We don't currently change the push ingestion sources in any of our envs
(prod, staging, prototype, test, dev) from their defaults. This
hardcodes those unchanging values and documents where to change them
should an engineer need to.
This reverts commit 1728dd306c (#3884), due to:
```
Pulse warning: queue "queue/treeherder-staging/jobs" is overgrowing
Warning: your queue "queue/treeherder-staging/jobs" on exchange "could not be determined" is
overgrowing (4083 ready messages, 4083 total messages).
...
```
Renaming as a separate commit since otherwise Git was struggling to
identify the renames, making the diff harder to review.
`index.rst` has been left unchanged, since we still want to use the
`toctree` directive for now.