doc: document flow for supporting type generation

doc/contributing/maintaining-types-for-nodejs.md

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+# Maintaining types for Node.js
+
+While JavaScript is an untyped language, there are a number of complementary
+technologies like [TypeScript](https://www.typescriptlang.org/) and
+[Flow](https://flow.org/) which allow developers to use types within their
+JavaScript projects. While many people don't use types, there are enough
+who do that the project has agreed it's important to work towards having
+[suitable types for end-users](https://github.com/nodejs/node/blob/master/doc/contributing/technical-priorities.md#suitable-types-for-end-users).
+
+## High level approach
+
+There are a number of ways that types could be maintained for Node.js ranging
+from shipping them with the Node.js runtime to having them be externally
+maintained.
+
+The different options were discussed as part of the
+[next-10](https://github.com/nodejs/next-10/blob/main/meetings/summit-nov-2021.md#suitable-types-for-end-users)
+effort and it was agreed that maintaining them externally is the best approach.
+Some of the advantages to this approach include:
+
+* Node.js maintainers do not need to be familiar with any given type
+  system/technology.
+* Types can be updated without requiring Node.js releases.
+
+The agreement was that the ideal flow would be as follows:
+
+* APIs are added/documented in the existing Node.js markdown files.
+* Automation in the Node.js project creates a machine readable JSON
+  representation of the API from the documentation.
+* Automation within external type projects consumes the JSON and automatically
+  generates a PR to add the API.
+
+## Generation/Consumption of machine readable JSON files
+
+When you run `make doc` the canonical markdown files used to
+document the Node.js APIs in the
+[doc/api](https://github.com/nodejs/node/tree/master/doc/api)
+directory are converted to both an `.html` file and a `.json` file.
+
+As part of the regular build/release process both the `html` and
+`json` files are published to [nodejs.org](https://nodejs.org/en/docs/).
+
+The generator that does the conversion is in the
+[tools/doc](https://github.com/nodejs/node/tree/master/tools/doc)
+directory.
+
+## Markdown structure
+
+The constraints required on the markdown files in the
+[doc/api](https://github.com/nodejs/node/tree/master/doc/api) directory
+in order to be able to generate the JSON files are defined in the
+[documentation-style-guide](https://github.com/nodejs/node/blob/master/doc/README.md#documentation-style-guide).
+
+## Planned changes (as of Jan 1 2022)
+
+While JSON files are already being generated and published, they are not
+structured well enough for them to be easily consumed by the type projects.
+Generally external teams need some custom scripts along with manual fixup
+afterwards.
+
+There is an ongoing effort to add additional markdown constraints and
+then update the flow in order to be able to generate a better
+JSON output.