Move ntpmon layer to https://github.com/canonical/ntpmon

[weiiwang01]

The upstream repositories (https://git.launchpad.net/ntpmon, https://github.com/paulgear/ntpmon) for ntpmon have removed all content related to the charm. I forked the upstream repository to the Canonical organization and reverted it to the last commit before the charm-related content was removed.

• [x] Add or update the JSON file for your layer, in the appropriate subdirectory (layers or interfaces)
• [x] Add an explicit docs URL field to your JSON, if appropriate. If omitted, it will default to the README.md at the root of your repo, but you may want to provide a more specific URL. If you're using the subdir field, then you definitely want to point to the README of the layer rather than the repo.
• [x] Run update_readme.py to update the user-friendly index in the README