Reset doc theme to ReadTheDocs' default theme

[ewjoachim]

On 2023, RTD made changes to the tools that are automatically installed. I'm pretty sure (though that might a false memory) that the Asgi doc used to use the rtd theme. Fact is that it now uses the default theme which looks so retro it belongs in a museum.

This PR resets the theme to RTD.
What this PR exactly does:

• Add a docs/requirements.txt file with docs dependencies
• Change the value for the theme

What this PR doesn't do but could (you tell me):

• Pin dependencies in docs/requirements.txt (I don't see dependabot or renovate being used in the repo so I'm wary of adding pinned deps, as they'll likely get old quick)
• Use a more modern theme such as Pradyun's excellent furo

I haven't changed the RTD build, and I see that this repo isn't configured to build docs in PRs. It should be as simple as checking this checkbox in the RTD admin, as long as github permissions are set correctly:

before

after