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/index.rst

@@ -0,0 +1,20 @@
+*******************************************
+chatmail relay documentation
+*******************************************
+
+.. image:: ../../www/src/collage-top.png
+   :target: https://testrun.org
+
+This documentation details how to setup, maintain and understand `chatmail <https://chatmail.at>`_ relays.
+
+Contributions and feedback welcome through the https://github.com/chatmail/relay repository.
+
+.. toctree::
+    :maxdepth: 5
+
+    getting_started
+    proxy
+    migrate
+    overview
+    related
+    faq