refactor README.rst and architecture file into sphinx doc project, automatically deploying on main merges and PRs.
* add FAQs from https://chatmail.at/relays landing page
* fix links, and streamline postfix/dovecot mentioning
* add linkcheck to CI, fix several links and streamlihne DKIM section while at it
* some streamlining, rename to "overview"
* ci: upload documentation to chatmail.at/doc/relay
* ci: main should be uploaded when docs.yaml changes
* ci: fix typo
* Update .github/workflows/docs-preview.yaml
Co-authored-by: missytake <missytake@systemli.org>
