Add API Documenter and generate API documentation for shared libs

[romainmenke]

I've enabled api-documenter which is build on top of the API analysis of api-extractor for all the shared libraries under packages/.

I've been playing with the documentation a bit but I don't want to write too much docs at this time.
I think it's better to first have a system for API docs and then work on content later.

see : #1201

interesting pages :

• overview of a package https://github.com/csstools/postcss-plugins/blob/documentation-for-packages--diligent-cheetah-7c39e64106/packages/css-tokenizer/docs/css-tokenizer.md
• large enum https://github.com/csstools/postcss-plugins/blob/documentation-for-packages--diligent-cheetah-7c39e64106/packages/css-tokenizer/docs/css-tokenizer.tokentype.md
• css token type https://github.com/csstools/postcss-plugins/blob/documentation-for-packages--diligent-cheetah-7c39e64106/packages/css-tokenizer/docs/css-tokenizer.token.md
• linking to other packages https://github.com/csstools/postcss-plugins/blob/documentation-for-packages--diligent-cheetah-7c39e64106/packages/cascade-layer-name-parser/docs/cascade-layer-name-parser.parsefromtokens.md

api-documenter markdown output results in a massive number of files for packages like the media query parser. This is a combination of the size of the API and that api-documenter makes a page for each bit of API. So a class field will have its own page.

I don't know why, but I expected there to be less output, but I guess this is mostly exposing existing complexity. Packages like the color helpers, tokenizer, parsers are just really large.

I am not sure if I have a strong opinion on this and want to see how it evolves.