Docs: Generate docusaurus sidebars from filesystem

[etrepum]

RE #5869 the configuration of docusaurus is fairly hard-coded

The packages list should be inferred from the filesystem (docs/files/*.md) rather than hard-coded in sidebars.js. They are all in alphabetical order anyway so we don't need any tricks.

While in there it may be useful to create a @lexical/react category in the API modules section so that they can be presented nicely (#5913). Not sure how much harder docusaurus-plugin-typedoc will make that (#5876 does clean up this part of the config, but I missed the hard-coded sidebar list).

I'll work on it this week, mostly just taking notes here.