[CI][Dev] Document how to maintain `CROSSBOW_GITHUB_TOKEN`

[kou]

Describe the enhancement requested

We're using CROSSBOW_GITHUB_TOKEN for comment bot:

arrow/.github/workflows/comment_bot.yml

Lines 51 to 58 in 54ea6d5

	- name: Handle GitHub comment event
	env:
	GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
	CROSSBOW_GITHUB_TOKEN: ${{ secrets.CROSSBOW_GITHUB_TOKEN }}
	run: |
	archery --debug trigger-bot \
	--event-name "${GITHUB_EVENT_NAME}" \
	--event-payload "${GITHUB_EVENT_PATH}"

We need to generate a token that has suitable scope (contents:write actions:write workflows:write, see also #dev > ✔ Update Crossbow tokens @ 💬) and ask INFRA to set it GitHub Actions secret.

We have token expiration notification mechanism: https://lists.apache.org/list?builds@arrow.apache.org:lte=3y:CROSSBOW_GITHUB_TOKEN

But I don't know how to configure it...

Component(s)

Continuous Integration, Developer Tools

[rok]

I was able to simply input CROSSBOW_GITHUB_TOKEN into https://github.com/apache/arrow/settings/secrets/actions and https://github.com/ursacomputing/crossbow/settings/secrets/actions. Is there something more I'm missing?

[kou]

Oh! I didn't notice that we can set our secrets by ourselves now! It's convenient!

[rok]

@kou is there another place we should put CROSSBOW_GITHUB_TOKEN into?