Document the use of GitHub automerge

[Mariatta]

Describe the enhancement or feature you'd like
We will begin using GitHub's automerge instead of miss-islington's automerge.
We need this to be documented on Devguide.

Additional context
See details and checklist here python/core-workflow#498 (comment)

A PR to miss-islington has been made to remove the automerge functionality.
Once we have the doc prepared, we can continue with the process.

[NishanthMuruganantham]

@hugovk @Mariatta I like to get my hands on it. Can you give me some more insights on this?

[Mariatta]

Here are the changes needed:

• Create a new section in https://devguide.python.org/core-developers/committing/ about automerging.

• in that new section, describe how it works, what it is (perhaps link to GitHub's own doc)

• in the same section, describe how to trigger it.

It seems like you're a first time contributor here. You may want to read up on how to contribute to the devguide here: https://devguide.python.org/documentation/devguide/

[methane]

I filed a new issue to python/core-workflow#501 about default commit message.

[hugovk]

@NishanthMuruganantham Hi, have you had a chance to look at this yet?