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/packages/maven.en-us.md

@@ -107,4 +107,4 @@ mvn install
 mvn install
 mvn deploy
 mvn dependency:get:
-```
+```

docs/content/doc/packages/pypi.en-us.md

@@ -82,4 +82,4 @@ pip install --index-url https://testuser:password123@gitea.example.com/api/packa
 ```
 pip install
 twine upload
-```
+```

docs/content/doc/packages/rubygems.en-us.md

@@ -124,4 +124,4 @@ gem install --host https://gitea.example.com/api/packages/testuser/rubygems test
 gem install
 bundle install
 gem push
-```
+```