Update the revision management how-to to use the charmcraft promote command

[lengau]

What needs to get done

manage-revisions.rst doesn't currently use the promote command, but rather only gets stuff from the charming-actions GH actions.

To wit:

• Wherever charming-actions is mentioned, mention promote command.
• Possibly replace charming-actions?

Why it needs to get done

New promote command. We should encourage using it!

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3989.

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[tmihoc]

It's basically this section: https://canonical-charmcraft.readthedocs-hosted.com/en/stable/howto/manage-revisions/#promote-a-charm-revision-to-a-better-risk-level

There are effectively two ways of doing this now, with the new way -- which come first -- being the promote command. (Note: The GH action way remains relevant for setting up a CI.)