fix: add docfx to the presubmit configuration and delete docs-presubmit

[chalmerlowe]

The Kokoro docs-presubmit check was failing. We have changed some things in the background related to our checks. This adds docfx to the presubmit.cfg, where we test docs.

🦕

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/