HDDS-11979. Fix heading and list checks in markdownlint.

[errose28]

What changes were proposed in this pull request?

The current markdownlint configuration has a typo that mixes the settings for headings and unordered lists:

• heading-increment uses sensible default values, it does not need to be added to the config
• heading-style should be specified as atx only
• ul-style should be dash. This config was incorrectly put under heading-increment

This was noticed during the review of #116, where * for lists was not flagged.

What is the link to the Apache Jira?

HDDS-11979

How was this patch tested?

Check off which of the following tests were done on this change. If additional testing was done, please elaborate here as well.

• The CI checks on my fork are passing
  • This run shows existing pages already conform to these rules
• I verified the rendered content using a local preview
• I manually verified the steps provided in this change work as described
  • This run adds a failing file and shows heading-style, heading-increment, and ul-style getting flagged

[adoroszlai]

Thanks @errose28 for the patch.