Update and refactor XQueue documentation

[sarina]

Refactors existing XQueue documentation to move technical implementation details to Site Ops docs.

Adds content from https://gist.github.com/UsamaSadiq/63084967fbe1a385bfdd260acaf25cb6 (with some editing) to the Site Ops docs.

Added or edited pages:

• https://docsopenedxorg--1413.org.readthedocs.build/en/1413/site_ops/how-tos/enable_xqueue.html
• https://docsopenedxorg--1413.org.readthedocs.build/en/1413/site_ops/references/xqueue_custom_watcher.html#xqueue-custom-watcher
• https://docsopenedxorg--1413.org.readthedocs.build/en/1413/site_ops/references/xqueue_external_grader.html#xqueue-reference
• https://docsopenedxorg--1413.org.readthedocs.build/en/1413/educators/concepts/exercise_tools/about_external_graders.html#about-external-grader-problems
• https://docsopenedxorg--1413.org.readthedocs.build/en/1413/educators/how-tos/course_development/exercise_tools/add_external_grader.html#add-an-external-grader-problem

[ormsbee]

@UsamaSadiq: One question for you regarding RabbitMQ vs. Redis.

[ormsbee]

@UsamaSadiq: Hasn't Tutor switched over to Redis for this?

[UsamaSadiq]

@ormsbee Good catch. I'm not familiar enough with current Tutor config but upon second look, yes Redis is being used by default now.
I had the old 2U notes in mind when reviewing this where both RabbitMQ or Redis were considered to be viable alternatives.

@sarina We would need to change the mention of RabbitMQ with Redis in the docs as per the latest standard.

[ormsbee]

Suggested change

	Tutor uses RabbitMQ by default.
	Tutor uses Redis by default.

[ormsbee]

Suggested change

	RabbitMQ should be listed.
	Redis should be listed.