add the ability to selectively hide (or annotate?) endpoints in the API doc page

Our API doc page currently lists all endpoints present in the Python module that are annotated with the relevant decorator, without any discrimination. But we are going to open them up selectively, with some of them being part of the code but not deployed in production yet (and hence protected by HTTP auth or other means). We need a way to differentiate these situations.

In particular:

• some endpoints will be implemented but we do not want their doc to be published on the page
  • → for this we can maybe just comment out the decorators in the code, although that might make the decorator code bitrot over time
• some endpoints will be implemented and ready, but not exposed and marked as "upcoming" (i'm thinking at endpoints backed by incomplete tables, for instance)
  • → for this it will be nice to have some sort of flashy annotation, like "coming soon", on the API doc page, and everything else typeset normally
  • if this is generic enough, in the future it could be used to also add annotations like "deprecated" to selected methods

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Migrated from T631 (view on Phabricator)