Add markdown link check to precommit hooks

[chriselion]

Automates part of the release process. Expecting this to fail in the short term; need to whitelist some things (like localhost:6006 for tensorboard)

[chriselion]

@ervteng @unityjeffrey Let me know what you think about this - to "officially" get the link checker running, we need to have the links from the localized docs point somewhere, because they doc they do point to hasn't been localized. I figure this is better since it's at least explicit in the markdown that it's linking somewhere outside the localization.

Alternative is just whitelist the localized docs and skip the checks on then.

[chriselion]

To run full check locally:
git ls-files | grep \.md$ | grep -v localized | xargs -n1 -t markdown-link-check -c markdown-link-check.config.json
This currently only checks a subset of files - pre-commit splits them into batches, but markdown-link-check only runs on the first file in the batch (because it doesn't take multiple files as arguments).

[chriselion]

Summary:

• Fixed broken links in Fix broken doc links #2327
• Will update release instructions with full commandline
• Blocked on either Add support for "xargs -n1" hooks? pre-commit/pre-commit#394 or Allow specifying multiple files on the command line tcort/markdown-link-check#78 to implement this in pre-commit right now.