Add SQL Scheduler docs

[matkuliak]

What's Inside

Documentation for the new feature "SQL Job Scheduler"

Preview

https://crate-cloud--59.org.readthedocs.build/en/59/reference/overview.html#failed-cloning (just below. why i'm linking to this explained below)

Highlights

• Introduction of the feature

• UI navigation to/around the feature. Explanation on the "Overview" and "Logs" tabs.

• Examples

  • Cleanup job - removes entries older than 30 days, runs once a day
  • Copying to another table job - copies from system table into user created table - runs every hour
  • Didn't find any other good examples, feel free to send any my way if 2 is not enough or they're not good enough

• Limitations and possible issues

• PR also contains few unrelated link fixes, leftovers from https://github.com/crate/cloud-docs/pull/55 that were not caught before. Also deletion of sign-up.rst, it complained about bunch of stuff and we don't need it anymore so deletion was easier.

Checklist

• [x] Link to issue this PR refers to (if applicable): https://github.com/crate/cloud/issues/1658
• [x] CLA is signed

[matkuliak]

Found one issue when I was going to link the header "SQL Scheduler". The consequence of this fix is that the header written this way can't be linked to. I will look for alternative, that should stop progress of this PR.