Split README into sphinx doc structured sections (#711)

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>

doc/source/related.rst

@@ -0,0 +1,20 @@
+
+Community developments
+======================
+
+Active development takes place in the `chatmail/relay github repository <https://github.com/chatmail/relay>`_.
+
+You can check out the `'chatmail' tag in the support.delta.chat forum <https://support.delta.chat/tag/chatmail>`_
+and ask to get added to a non-public support chat for debugging issues.
+
+We know of two work-in-progress alternative implementation efforts:
+
+-  `Mox <https://github.com/mjl-/mox>`_: A Golang email server. `Work
+   is in progress <https://github.com/mjl-/mox/issues/251>`_ to modify
+   it to support all of the features and configuration settings required
+   to operate as a chatmail relay.
+
+-  `Maddy-Chatmail <https://github.com/sadraiiali/maddy_chatmail>`_: a
+   plugin for the `Maddy email server <https://maddy.email/>`_ which
+   aims to implement the chatmail relay features and configuration
+   options.