Add markdownlint (#20512)

Add `markdownlint` linter and fix issues. Config is based on the one
from electron's repo with a few rules relaxed.

docs/content/doc/developers/api-usage.en-us.md

@@ -48,7 +48,6 @@ A new token can be generated with a `POST` request to
 Note that `/users/:name/tokens` is a special endpoint and requires you
 to authenticate using `BasicAuth` and a password, as follows:
 
-
 ```sh
 $ curl -XPOST -H "Content-Type: application/json"  -k -d '{"name":"test"}' -u username:password https://gitea.your.host/api/v1/users/<username>/tokens
 {"id":1,"name":"test","sha1":"9fcb1158165773dd010fca5f0cf7174316c3e37d","token_last_eight":"16c3e37d"}