Add more Checkstyle checks for AsciiDoc files

As a continuation of #257 (closed) where we enabled Checkstyle for AsciiDoc files, we should enable more checks:

See also:

• !217 (comment 369972)

Check to consider:

• Things we come up in issue #254, where we want to document the guidelines.
• [source, not followed by a space.
• Headings: The first heading in a file should be level 2. All subsequent ones should be higher levels. Level increase should be from 2 to 3 to 4, not 2 to 4, although 4 to 2 is allowed. This should be able to handle :leveloffset: +1 etc as well.
• Require link: for all links. So no URLs without link:.
• ...