Use GH actions to auto-pull the manual's changes straight to server

[pjrobertson]

As the title says. It seems like we'd just need to set up a GH action to run what's listed in the readme here: https://github.com/quicksilver/manual#updating-the-live-manual

I'm going to ping @n8henrie once more for GH action magic you can pull off ;-)

[n8henrie]

Assigning myself -- kind of nice being able to use the search feature to filter these out: assignee:@me is:open is:issue

[n8henrie]

I think we're in business: https://github.com/quicksilver/manual/actions

I still have the --dry-run in place for both this and qsapp.com, let me know if you think it looks ready to merge to main and I'll cut the PR for both repos.

Also adding a filter so it only runs the deploy script on pushed to main, so we can still fiddle with feature branches.