[Docs] Splitting the sphinx-based Arrow docs into separate sphinx projects

[asfimport]

See the mailing list (https://mail-archives.apache.org/mod_mbox/arrow-dev/202112.mbox/%3CCALQtMBbiasQtXYc46kpw-TyQ-TQSPjNQ5%2BkoREuKvJ3hJSdWjw%40mail.gmail.com%3E) and this google doc (https://docs.google.com/document/d/1AXDNwU5CSnZ1cSeUISwy_xgvTzoYWeuqWApC8UEv97Q/edit?usp=sharing) for more context.

Reporter: Joris Van den Bossche / @jorisvandenbossche

PRs and other links:

• GitHub Pull Request #11980

_{Note: This issue was originally created as ARROW-15117. Please see the migration documentation for further details.}

[asfimport]

Apache Arrow JIRA Bot:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned per project policy. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.

[AlenkaF]

Just to note, I have created a new issue for alternative easy first solution: #35609
The PR: #35610

[AlenkaF]

Closing in favour of #35609