Merging stubs into single files

[jayercule]

Description

Merging the stub files for api/administration/, migrate/pg-dump-and-restore/, and mst-aiven-client into single files.

Links

N/A

Writing help

For information about style and word usage, see the style guide

Review checklists

Reviewers: use this section to ensure you have checked everything before approving this PR:

Subject matter expert (SME) review checklist

• [ ] Is the content technically accurate?
• [ ] Is the content complete?
• [ ] Is the content presented in a logical order?
• [ ] Does the content use appropriate names for features and products?
• [ ] Does the content provide relevant links to further information?

Documentation team review checklist

• [ ] Is the content free from typos?
• [ ] Does the content use plain English?
• [ ] Does the content contain clear sections for concepts, tasks, and references?
• [ ] Have any images been uploaded to the correct location, and are resolvable?
• [ ] If the page index was updated, are redirects required and have they been implemented?
• [ ] Have you checked the built version of this content?

[CLAassistant]

All committers have signed the CLA.

[billy-the-fish]

@jayercule, really like this PR, really improves the doc navigation. @jamessewell, I've updated the links and navigation for this PR. Looks good locally. Not too happy about merging to master without checking on dev.

[billy-the-fish]

I hope nobody minds, but I have moved this to https://github.com/timescale/docs/pull/3013 so I can review the link and navigation updates. Being a bit defensive as this is my first PR with navigation changes.