Add missing FDB API docs

[caraghbiner]

Description

Add documentation for some undocumented FDB API functions.

Some initial comments:

• WIP. First draft is AI generated, and will be edited by a human, because it frankly isn't very good (or accurate). PR will remain as draft until then.
• I think some of the other API functions could use a touch up also. e.g. list currently has "List data present at the archive [...]" which does not make sense.
• The goal is to write something just "good enough" – mediocre (but not bad) documentation is better than none. Each of these API functions could have an entire page of examples and explanations in the docs, but that is not in scope of this PR.

For tickets FDB-458/FDB-459/FDB-460/FDB-461/FDB-462/FDB-463.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌈🌦️📖🚧 Documentation FDB 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/fdb/pull-requests/PR-324