docs: Auto-format markdown documentation

This patch uses mdformat to auto-format markdown documentation.

To support \ for line breaks (which is what mdformat uses), mkdocs now
requires the pymdownx.escapeall extension.

Because of incompatibilities between mdformat (which formats to
CommonMark) and mkdocs (which expects Python-Markdown), there are now
some limitations: no nested lists, multi-paragraph list items, or
admonitions support.

These limitations are acceptable for now, although some of the docs had
to be adjusted to meet them.

docs/clients.md

@@ -1,18 +1,15 @@
-
 # Clients
 
 chasquid supports most SMTP clients, but requires them to have some features:
 
-- Support TLS (either
-  [STARTTLS](https://datatracker.ietf.org/doc/html/rfc3207) or
-  [implicit TLS](https://datatracker.ietf.org/doc/html/rfc8314#section-3.3))
+- Support TLS (either [STARTTLS](https://datatracker.ietf.org/doc/html/rfc3207)
+  or [implicit TLS](https://datatracker.ietf.org/doc/html/rfc8314#section-3.3))
 - Support the
   [PLAIN authentication method](https://datatracker.ietf.org/doc/html/rfc4954#section-4).
 
 All modern clients should support both, and thus have no problems talking to
 chasquid.
 
-
 ## Configuration examples
 
 ### [msmtp](https://marlam.de/msmtp/)
@@ -42,13 +39,10 @@ password SECRET
 Replace the `SERVER`, `USER@DOMAIN` and `SECRET` strings with the appropriate
 values.
 
-
 ## Problematic clients
 
 These clients are known to have issues talking to chasquid:
 
-- [ssmtp](https://packages.debian.org/source/unstable/ssmtp): does not
-  support the PLAIN authentication method. It is also unmaintained.  
+- [ssmtp](https://packages.debian.org/source/unstable/ssmtp): does not support
+  the PLAIN authentication method. It is also unmaintained.\
   Please use [msmtp](https://marlam.de/msmtp/) instead.
-
-