Adds NPM article. (#22942)

* Adds NPM article.

Update glossary.md, doc-links.yaml with links to new entry.

* Update docs/docs/glossary.md

Follow capitalization of "Node package manager" from npmjs.com.

Co-Authored-By: Aisha Blake <aisha@gatsbyjs.com>

* Update docs/docs/glossary/npm.md

Update document title to use capitalization preferred by npm, Inc.

Co-Authored-By: Aisha Blake <aisha@gatsbyjs.com>

* Update docs/docs/glossary/npm.md

Change capitalization of "Node Package Manager" to "Node package manager"

* Update docs/docs/glossary/npm.md

Update directions for installing plugins to remove the `--save` flag
since npm now does this by default.

Change tutorial link to be relative instead of absolute.

* Update docs/docs/glossary/npm.md: Clarify global installation

Remove language about Gatsby "needing" to be installed globally.
Instead call it the recommended way. Explain npx a little more.
Added a clarifying note about plugins, npx and npm install.

* Update docs/docs/glossary.md

Capitalization of npm

Co-Authored-By: Aisha Blake <aisha@gatsbyjs.com>

* Update docs/docs/glossary/npm.md

Use active voice re: installing Gatsby globally.

Co-Authored-By: Aisha Blake <aisha@gatsbyjs.com>

* Update docs/docs/glossary/npm.md

Change NPM to npm for style, consistency.

Co-Authored-By: Aisha Blake <aisha@gatsbyjs.com>

Co-authored-by: Aisha Blake <aisha@gatsbyjs.com>

docs/docs/glossary.md

@@ -220,9 +220,9 @@ A way of writing HTML content with plain text, using special characters to denot
 
 ## N
 
-### NPM
+### [npm](/docs/glossary/npm)
 
-[Node](#node) [Package](#package) Manager. Allows you to install and update other packages that your project depends on. [Gatsby](#gatsby) and [React](#react) are examples of your project's dependencies. See also: [Yarn](#yarn).
+[Node](#node) [package](#package) manager. Allows you to install and update other packages that your project depends on. [Gatsby](#gatsby) and [React](#react) are examples of your project's dependencies. See also: [Yarn](#yarn).
 
 ### Node
 

docs/docs/glossary/npm.md

@@ -0,0 +1,59 @@
+---
+title: npm or Node package manager
+disableTableOfContents: true
+---
+
+Learn what _npm_ is, how to use it, and how it fits in to the Gatsby ecosystem.
+
+## What is npm?
+
+<abbr>
+  npm
+</abbr>, or Node package manager, is the default package manager for the [Node.js](/docs/glossary/node) JavaScript runtime. It lets you install and update libraries and frameworks (dependencies) for Node-based projects, and interact with the npm Registry. You'll use npm to install and upgrade Gatsby and its plugins.
+
+npm is a [command line](/docs/glossary#command-line) tool. You'll need Terminal (Mac, Linux) or Command Prompt (Windows) in order to run its commands. To use one of npm's features, type `npm <command>` . For example, `npm help` displays a list of available features, including `install`, `uninstall`, `update`, and `search`.
+
+npm is installed alongside Node during the default [installation process](/tutorial/part-zero/#install-nodejs-for-your-appropriate-operating-system). You don't need to take any additional steps to add it to your environment.
+
+### Using npm to install Gatsby
+
+You'll need to install Gatsby globally to use Gatsby CLI commands such as `gatsby new`. To do so, use `npm install` with the `--global` or `-g` flag.
+
+```shell
+npm install -g gatsby-cli
+```
+
+Once the installation completes, you can run `gatsby new my-project` to create a new Gatsby project.
+
+### Using npx to install Gatsby
+
+> **Note:** `npx` requires npm version 5.2 or later. If you've installed the latest versions of Node and npm, you should also have npx. Otherwise, you should upgrade Node and/or npm.
+
+You can also use [npx](https://www.npmjs.com/package/npx) to install Gatsby. npx ships with npm. It allows you to install a package and run a command in one step. For example, instead of running `npm install -g gatsby-cli` then `gatsby new my-project`, you could use the following command.
+
+```shell
+npx gatsby new my-project
+```
+
+This will download and install the latest version of Gatsby, then create a new Gatsby project in the `my-project` folder. Choosing this method will not make the Gatsby CLI globally available, however. If you install Gatsby using npx, you'll need to use `npx gatsby` or `npm run` to execute Gatsby commands, e.g.: `npx gatsby develop` or `npm run develop`.
+
+### Using npm to install Gatsby plugins
+
+Gatsby has a robust collection of [plugins](/plugins/) that add functionality or data sourcing to your Gatsby sites. Adding a plugin as a project dependency uses the same process as installing Gatsby itself. Use `npm install <package-name>`. To add the [gatsby-source-filesystem](/packages/gatsby-source-filesystem), plugin, for example, you'd use the following command.
+
+```shell
+npm install gatsby-source-filesystem
+```
+
+> **Note:** Use `npm install` to add plugins, even if you installed Gatsby using npx.
+
+> **Note:** You'll still need to update `gatsby-config.js` to add the plugin's functionality to your site.
+
+This will update the dependencies list of `package.json` and `package-lock.json`. Commit both files to your project's repository. Doing so makes it easy to keep your Gatsby project consistent across team members and computers. When another team member clones your repository, they can use `npm install` to install the dependencies included in `package-lock.json`.
+
+### Learn more about npm
+
+- [npm](https://www.npmjs.com/) official website
+- [Node.js](https://nodejs.org/en/) official website
+- [An introduction to the npm package manager](https://nodejs.dev/an-introduction-to-the-npm-package-manager) from Nodejs.dev
+- [Set Up Your Development Environment](/tutorial/part-zero/) from the Gatsby docs

www/src/data/sidebars/doc-links.yaml

@@ -784,6 +784,8 @@
           link: /docs/glossary/mdx/
         - title: Node.js
           link: /docs/glossary/node/
+        - title: npm
+          link: /docs/glossary/npm/
         - title: React
           link: /docs/glossary/react/
         - title: Server Side Rendering