Lint all markdown files in CI

[kleimkuhler]

Motivation

linkerd/rfc#22

Solution

Use the markdown-lint-action to lint all .md files for all pull requests
and pushes to master.

This action uses the default rules outlined in markdownlint
package.

The additional rules are added are explained below:

• Ignore line length lints for code blocks
• Ignore line length lints for tables
• Allow duplicate sub-headers in sibling headers (e.g. allowing multiple ##
  Significant headers in CHANGES.md as long as they are part of separate
  release headers)

Signed-off-by: Kevin Leimkuhler kevin@kleimkuhler.com

[kleimkuhler]

This depends on #4403 which applies all the lints that this will error on.

[grampelberg]

LGTM, should we have a .prettierrc in here to do the formatting by default for folks?

[adleong]

Is there a way to run markdown linting locally? How do I know if I've violated a lint rule before running through CI?

[kleimkuhler]

@adleong I answered this here.

I added a bin/markdownlint script here that will use npm to install the CLI command locally--not globally. I tried downloading the GH release and using node to run the JS file, but it's missing a few things that prevent it from being run out of the box.

Using npm install markdownlint@version makes the most sense.

[alpeb]

LGTM 👍