docs: Detail front-end development instructions (apache#19870)

* chore: Beautify CONTRIBUTING

Automatically done using the Markdown-All-In-One extension for VSCode:
https://open-vsx.org/extension/yzhang/markdown-all-in-one

Signed-off-by: Étienne Boisseau-Sierra <etienne.boisseau-sierra@unipart.io>

* docs: Detail front-end development instructions

Based on the silly error that led to [this
discussion](https://apache-superset.slack.com/archives/C014LS99C1K/p1650969237848999),
we clarify that when modifying the front-end locally, one should access
the Webpack dev servers, rather than Flask directly, to see the latest
changes take effect.

Signed-off-by: Étienne Boisseau-Sierra <etienne.boisseau-sierra@unipart.io>

* docs: Underline typical workflow

Signed-off-by: Étienne Boisseau-Sierra <etienne.boisseau-sierra@unipart.io>

CONTRIBUTING.md

@@ -55,6 +55,7 @@ little bit helps, and credit will always be given.
  - [Images](#images)
  - [Flask server](#flask-server)
  - [OS Dependencies](#os-dependencies)
+ - [Dependencies](#dependencies)
  - [Logging to the browser console](#logging-to-the-browser-console)
  - [Frontend](#frontend)
  - [Prerequisite](#prerequisite)
@@ -68,11 +69,13 @@ little bit helps, and credit will always be given.
  - [Feature flags](#feature-flags)
  - [Git Hooks](#git-hooks)
  - [Linting](#linting)
+ - [Python](#python)
+ - [TypeScript](#typescript)
  - [Conventions](#conventions)
- - [Python](#python-conventions)
+ - [Python Conventions](#python-conventions)
  - [Typing](#typing)
- - [Python](#python-typing)
- - [TypeScript](#typeScript-typing)
+ - [Python Typing](#python-typing)
+ - [TypeScript Typing](#typescript-typing)
  - [Testing](#testing)
  - [Python Testing](#python-testing)
  - [Frontend Testing](#frontend-testing)
@@ -92,7 +95,7 @@ little bit helps, and credit will always be given.
  - [SQL Lab Async](#sql-lab-async)
  - [Async Chart Queries](#async-chart-queries)
  - [Chart Parameters](#chart-parameters)
- - [Datasource & Chart Type](#datasource--chart-type)
+ - [Datasource \& Chart Type](#datasource--chart-type)
  - [Time](#time)
  - [GROUP BY](#group-by)
  - [NOT GROUPED BY](#not-grouped-by)
@@ -568,7 +571,22 @@ There are three types of assets you can build:
 
 #### Webpack dev server
 
-The dev server by default starts at `http://localhost:9000` and proxies the backend requests to `http://localhost:8088`. It's possible to change these settings:
+The dev server by default starts at `http://localhost:9000` and proxies the backend requests to `http://localhost:8088`.
+
+So a typical development workflow is the following:
+
+1. [run Superset locally](#flask-server) using Flask, on port `8088` — but don't access it directly,<br/>
+ ```bash
+ # Install Superset and dependencies, plus load your virtual environment first, as detailed above.
+ FLASK_ENV=development superset run -p 8088 --with-threads --reload --debugger
+ ```
+2. in parallel, run the Webpack dev server locally on port `9000`,<br/>
+ ```bash
+ npm run dev-server
+ ```
+3. access `http://localhost:9000` (the Webpack server, _not_ Flask) in your web browser. This will use the hot-reloading front-end assets from the Webpack development server while redirecting back-end queries to Flask/Superset: your changes on Superset codebase — either front or back-end — will then be reflected live in the browser.
+
+It's possible to change the Webpack server settings:
 
 ```bash
 # Start the dev server at http://localhost:9000