Split README into sphinx doc structured sections

[hpk42]

The original text from README.md has not been much modified (it was translated using pandoc + some manual formatting fixes).

You can see a rendered version of the draft doc here: https://merlinux.eu/chatmail-docs/

• add instructions how to setup sphinx
• remove/shorten README (see https://github.com/chatmail/relay/tree/hpk/chatmail-docs?tab=readme-ov-file )
• CI for generating docs to staging.chatmail.at (basic workflow ci.yml exists but does not work due to secrets problems)
• CI for pushing to chatmail.at/doc/relay on PR merge
• move FAQ from https://chatmail.at/relays to an FAQ page in these docs and remove from chatmail.at. See https://github.com/chatmail/pages/pull/21 for removal of the FAQ-style sections.

[missytake]

Some thoughts:

The icon could be white in dark mode, maybe https://stackoverflow.com/a/79291623 is enough to fix this?

The ToC on index.rst is a bit hard to read, maybe we should adjust the css a bit?

The graphic from architecture.rst would fit well into overview.rst I think :)

I feel like troubleshooting isn't a fitting headline for touch /etc/chatmail-nocreate... maybe we could call it customization and move "Modifying the home page", "Refining the web pages", and "Custom web pages" there, who are right now at getting_started?

[hpk42]

the intro page has changed completely, also new sectioning since you last looked. See top-level PR description for status.

[missytake]

Sorry, will put my CI commits in #724

[github-actions]

Check out the page preview at https://staging.chatmail.at/doc/relay/711/

[missytake]

Looks good! Let's iterate over things like darkmode, content, and structure :)