Serve multiple versions of API docs

[dessalines]

@Nothing4You mentioned this: we should try to serve up multiple versions of API docs. For now, I think

• A next (which uses lemmy-js-client#main)
• A latest (which uses the latest release tag)

These could then be served up at /api/next, and /api/latest

Context #451

[Nothing4You]

If possible it would be nice to have each release kept, latest or stable could point to the latest release and then next from the main branch.
This is a common setup e.g. on readthedocs pages.
See for example https://docs.aiohttp.org/en/stable/, on the bottom right you can switch between a variety of versions:

[dessalines]

I'm not sure how they're doing that, but our docs are built from specific lemmy-js-client submodule declarations, so it probably wouldn't be a great idea to add a ton of these to the repo.

I've added just the two mentioned above.