Improve markdown files and add markdownlint

[waldyrious]

• Normalize headings in markdown files: start each file with a h1-level heading, and increment levels for section headings one at a time. These correspond to rules MD041 and MD001 of markdownlint, respectively.
• Assorted improvements to markdown files:
  • Add introductory context
  • Reword the introductory paragraphs of GOVERNANCE.md into a proper section
  • Fix a typo in CONTRIBUTING.md
  • Apply various fixes recommended by markdownlint
  • Change list syntax in markdown files (use dashes for lists, and leave asterisks for emphasis, as first proposed here)
• Add a GitHub Action to run markdownlint on markdown files in pull requests

[Xymph]

Good improvements, with some nice touches like the text type for the Array example.
One more tweak: https://github.com/hamstar/Wikimate/blame/503625f9e2c23a442d8bda368c4974281c2bd1f3/GOVERNANCE.md#L46
Change "the README" into "README.md"

[Xymph]

General question: how did you add the remarks below the commit message, e.g. "Use dashes for lists..." ?

[waldyrious]

One more tweak: https://github.com/hamstar/Wikimate/blame/503625f9e2c23a442d8bda368c4974281c2bd1f3/GOVERNANCE.md#L46
Change "the README" into "README.md"

Done :)

General question: how did you add the remarks below the commit message, e.g. "Use dashes for lists..." ?

A git commit message can have a summary and a body, though most of the time only the former is used, which leads to the conflation of the expression "commit message" with the summary alone. But, as I alluded to recently, there is a recommended maximum length for the summary, so in many cases it's useful to provide additional details in the body.

In practice, the way you add content to the commit message body is by making a multi-line commit message, with a blank line separating the summary and the body. In the command line, simply typing enter before closing the quotes will allow you to add additional lines to the string:

$ git commit -m "Lorem ipsum↵
> ↵
> Dolor sit amet, consectitur adipisci elit."↵

GitHub will display commit message bodies as expandable notes with the "..." sign, as you see above.

[Xymph]

And that's another wrap. 👍