Add GH action to lint markdown files

[kke]

Signed-off-by: Kimmo Lehto [email protected]

Adds a Markdown linter into github actions and fixes linting errors to make it pass.

• Almost all of the markdown files contained trailing whitespaces on half of the lines, causing any doc PRs to contain unrelated changes when saved with an editor that removes them.
• There were a couple of runaway code blocks without closing backticks.
• There were ordered lists where numbers were repeated or missing from between
• There were incorrectly rendering ordered lists because of indentation
• There were hard tabs
• There were multiple sequential blank lines
• Blank lines around headers, tables, lists and code blocks were randomly there or not
• There were multiple cases of skipping heading levels (from ## straight to #### without ### in between, heading used to create bold?)
• Several code/yaml blocks were missing the language tag after backticks
• The command example code blocks were normalized: when command examples display output, the command is prefixed with $, when they do not display any output, the $ is omitted to ease copypasting)

These are all things the linter will check for.

It was a lot of trouble to make it pass, but I'd say it's worth it because obvious mistakes were catched and will be automatically catched from now on (also forced linting makes diffs smaller).

[jasmingacic]

looks legit