docs: merge postgrest-docs into the core repository

[laurenceisla]

Closes #2814

• Merge documentation content into the core repo. Reference: Manually update changelog for v8 release #1897 (comment)
• Merge GitHub CI
• Merge Nix tooling
• Configure Read the Docs

[steve-chavez]

Some extra things to consider:

• The docs/default.nix and docs/shell.nix should be merged with the root ones. The docs have a postgrest-docs-* prefix so this should work ok, no clashes.
• The readme and the docs should share the sponsor logos, right now they're duplicated.

[laurenceisla]

The readme and the docs should share the sponsor logos, right now they're duplicated.

I think it should be done in a different PR after merging, since a new Sponsor was added. Kinda scary to handle rebase conflicts here.

---

I tested a Read The Docs deploy using this branch and it seems to be working without issues:

https://postgrest-test.readthedocs.io/en/stable/

[laurenceisla]

@wolfgangwalther WDYT? Did you manage to compare both strategies to merge the docs into the core repo?

[wolfgangwalther]

Did you manage to compare both strategies to merge the docs into the core repo?

I did not, yet. It's on my list after #3169 and some more experiments with cross-compiling.

[wolfgangwalther]

As a first step, I rebased this PR on the recent merge of the docs main branch. Will still go through the details of the nix setup etc. later.

[wolfgangwalther]

Will still go through the details of the nix setup

Done.

The readme and the docs should share the sponsor logos, right now they're duplicated.

Done.

Will wait for CI to pass and then merge.