[SUPPORTENG-916] Updating a few links to match platform docs

[robertscl]

This PR updates a few broken links to match them with the equivalents already in the platform's CLI docs (https://platform.zapier.com/cli_docs/docs).

It also includes a new note, which was already in README-source.md but wasn't pushed to other files in someone else's earlier commit.

Note, I had some trouble with the pre-commit hook, so I ran yarn docs and then staged/committed individual files. Please let me know if anything here looks incorrect and I'll fix re-submit.

[rnegron]

Looks good, thank you for making this PR!

Side note to myself: The .html files are being modified by our existing tooling but should probably be removed after making sure no other documentation we have is actually pointing to the websites they serve:

• docs/index.html serves https://zapier.github.io/zapier-platform/ (superseded by https://platform.zapier.com/cli_docs/docs)
• docs/cli.html serves https://zapier.github.io/zapier-platform/cli.html (superseded by https://platform.zapier.com/cli_docs/cli)