fix(ldoc): set `format` to "markdown" (#3500)

The CI currently doesn't install (correctly?) the "discount" formatter,
so ldoc fallbacks to the default "markdown" formatter. However, if we
try to install the discount or lua-discount, the doc rendering is
slightly different from what we currently have.

All of this was already mentioned in the issue "Generate apidoc doesn't
use the discount markdown formatter #3500".

This change removes the configuration in the `config.ld` file, so we
don't have the "format: using built-in markdown" message any longer in
in the CI logs.

Note that I also removed the lua-discount installation from the
`apidoc.yml` workflow, since it wasn't working. (I guess it installs the lua
package for a different version of lua than the one used by ldoc)

.github/workflows/apidoc.yml

@@ -74,7 +74,6 @@ jobs:
             libxkbcommon-dev \
             libxkbcommon-x11-dev \
             xutils-dev \
-            lua-discount \
             liblua5.3-dev \
             lua5.3
 

docs/config.ld

@@ -9,7 +9,6 @@ project='awesome' -- luacheck: globals project
 title='awesome API documentation'  -- luacheck: globals title
 description='API documentation for awesome, a highly configurable X window manager (version @AWESOME_VERSION@).'  -- luacheck: globals description
 
-format='discount'  -- luacheck: globals format
 dir='../doc'  -- luacheck: globals dir
 
 -- Make the docs prettier