docs: enable a few more markdownlint rules and fix violations

[bmish]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[X] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Enables a few useful / easy / trivial / recommended markdownlint rules that were disabled:

• https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md001---heading-levels-should-only-increment-by-one-level-at-a-time
• https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md012---multiple-consecutive-blank-lines (autofixable)
• https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md034---bare-url-used (autofixable)
• https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md040---fenced-code-blocks-should-have-a-language-specified

This also ensures that yarn markdownlint . runs successfully.

Also includes small upgrades of markdownlint dependencies (no immediate impact to us):

• https://github.com/igorshubovych/markdownlint-cli/releases/tag/v0.30.0
• https://github.com/igorshubovych/markdownlint-cli/releases/tag/v0.29.0
• https://github.com/DavidAnson/markdownlint#history

Is there anything you'd like reviewers to focus on?

No.

[nzakas]

LGTM. Thanks