[CI] Auto add license headers to all Markdown files with pre-commit

[jbampton]

Did you read the Contributor Guide?

• Yes, I have read the Contributor Rules and Contributor Development Guide

Is this PR related to a JIRA ticket?

• No:
  • this is a CI update. The PR name follows the format [CI] my subject

What changes were proposed in this PR?

Let us automate all the things :)

Some other PRs like this one have already been merged.

How was this patch tested?

Ran pre-commit run --all-files both before and after the license headers were added.

After the license headers were added to the Markdown files I ran:

mkdocs serve

and browsed the generated site locally doing a visual inspection of some of the web pages.

Did this PR include necessary documentation updates?

• No, this PR does not affect any public API so no need to change the documentation.

[jiayuasu]

Can you address the conflict? @jbampton