Add workflow to automate manpage updates

[nandahkrishna]

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This PR aims to add a workflow to automate manpage (and completions) updates. Contributors usually run brew man and commit any changes themselves, but automating this step is desirable. I have minimal experience with workflows, but I've based this almost entirely on sorbet.yml.

Currently, I've set the workflow to run on pushes except for specific paths (that would be changed by running brew man), but this could be changed if something else is more apt.

[BrewTestBot]

Review period will end on 2021-02-19 at 07:13:24 UTC.

[nandahkrishna]

I've updated the workflow to run on:

• schedule (cron)
• workflow_dispatch (manually)
• push to branches (in this repository) except master that modify the workflow file
• pull_request that modify files which could lead to manpage updates.

I'm sure this PR needs to be reviewed/modified a little more, so I've not combined all the scheduled jobs into one scheduled.yml just yet.

[BrewTestBot]

Review period ended.

[nandahkrishna]

The workflow has been updated to run on:

• schedule: once a day
• pushes to master that affect specific paths, which could result in manpage updates.

[Rylan12]

Thanks for the clarification, Nanda and Mike! I agree, I think the current setup makes sense.

This looks good to me. I haven't tested it out, of course, but I'd be comfortable merging to give it a go.

[nandahkrishna]

Thanks everyone!

Update: First run