I would rather use Spotless for these checks, for consistency with other
format checkers that we already use.  Spotless supports Markdown via
flexmark, but unfortunately [Spotless does not provide access to any of
flexmark's configuration
options](diffplug/spotless#2183).  That would
leave us with a Markdown linter that is too lenient for my tastes.  For
example, [Spotless's flexmark configuration does not even restrict
maximum line length](diffplug/spotless#2167).

Implements wala#818.

I would rather use Spotless for these checks, for consistency with other
format checkers that we already use.  [Spotless supports Markdown via
flexmark](https://github.com/diffplug/spotless/tree/main/plugin-gradle#flexmark),
but unfortunately [Spotless does not provide access to any of flexmark's
configuration
options](diffplug/spotless#2183).  That would
leave us with a Markdown linter that is too lenient for my tastes.  For
example, [Spotless's flexmark configuration does not even restrict
maximum line length](diffplug/spotless#2167).

Resolves wala#818.

I would rather use Spotless for these checks, for consistency with other
format checkers that we already use.  [Spotless supports Markdown via
flexmark](https://github.com/diffplug/spotless/tree/main/plugin-gradle#flexmark),
but unfortunately [Spotless does not provide access to any of flexmark's
configuration
options](diffplug/spotless#2183).  That would
leave us with a Markdown linter that is too lenient for my tastes.  For
example, [Spotless's flexmark configuration does not even restrict
maximum line length](diffplug/spotless#2167).

Resolves #818.