[CI] Add markdown check for normalized code-blocks + .md page fixes

[chalin]

• Contributes to CI improvements - 2023/08 #3122
• This PR is best reviewed by ignoring whitespace changes
• Adds an NPM script named fix:markdwon which will run markdownlint in "fix" mode
• Adds a custom markdownlint rule, named trim-code-block-and-unindent, to check and optionally clean up fenced code blocks of:
  • Excessive blank lines at the start or end of the code
  • Unnecessary indentation
• Adds trim-code-block-and-unindent:true and no-trailing-spaces:true to the markdownlint config
• Fixes fenced code blocks in our docs using the new rule
• Notes:
  • Custom rules are controlled in the same way as other rules. So they can be turned on/off for the project or selectively within a file.
  • The added custom rule is a bit hacky (I didn't quite understand how markdownlint rules worked when I started 🤷🏼‍♂️), but it works so I'm committing it now. I'll rework the rule later.

[cartermp]

Hmmm. What is "unecessary indent" here for code blocks? Maybe a sample would help demonstrate. I don't understand offhand how this accounts for whitespace-sensitive languages.

[chalin]

Maybe "excessive indentation" would be a better term than "unnecessary indentation"?

Here's an example from the PR:

The algorithm will shift the code in the code block to the left as much as possible while preserving the relative indentation of non-whitespace characters. Of the 25+ files in this PR that were fixed, do you see any that were incorrectly adjusted (I've re-enabled view-whitespace diffs).

[cartermp]

Okay, thanks, makes sense. So long as relative indentation is preserved then this is good.