Add documentation around queue monitoring

[mattrobenolt]

See GH-2809

Changes proposed in this pull request:

• Add documentation to support the current iteration of queue monitoring
• Moves docs/health-check into a more general, docs/monitoring

@getsentry/infrastructure

[codecov-io]

Current coverage is 82.58%

Merging #2944 into master will decrease coverage by -0.43% as of 5d65b83

@@            master   #2944   diff @@
======================================
  Files          918     918       
  Stmts        35715   35715       
  Branches         0       0       
  Methods          0               
======================================
- Hit          29649   29496   -153
  Partial          0       0       
- Missed        6066    6219   +153

Review entire Coverage Diff as of 5d65b83

---

Uncovered Suggestions

1. +0.07% via ...try/utils/apidocs.py#432...454
2. +0.07% via ...try/utils/apidocs.py#117...139
3. +0.06% via ...y/models/dsymfile.py#79...99
4. See 7 more...

Powered by Codecov. Updated on successful CI builds.

[tkaemming]

s/retuns/returns/