[Doc] Document the different subcomponents that make up the CI and how they fit together

[asfimport]

High-level overview:

• Possibly add diagram: https://lucid.app/lucidchart/ea221ae9-afc5-4453-8939-970e08759127/edit?shared=true&page=0_0#

• Short description of each component

  Specific things to add:

• when you commit to an open PR (e.g. also the automatic scripts we run that check the title etc)

• what this workflow is about and why it exists and when runs: https://github.com/apache/arrow/blob/master/.github/workflows/cpp_cron.yml

• what happens when we merge a PR (e.g. the dev PR)

• basically, what triggers and when they happen, then once that's documented, it makes a lot more sense for people to know what to look at

  Generally add more content to make it easier to get started with understanding CI.

  This section is the relevant location: https://github.com/apache/arrow/tree/master/docs/source/developers

  Potentially add something about how linx-apt-r (etc) depend on other dockerfiles

Reporter: Nicola Crane / @thisisnic
Assignee: Nicola Crane / @thisisnic

PRs and other links:

• GitHub Pull Request #11821

_{Note: This issue was originally created as ARROW-13841. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 11821
#11821