docs: reference (API) documentation coverage to 100%

[lostmygithubaccount]

Currently, many user-facing Ibis APIs are missing documentation in the "Reference" section. Some examples:

• unpack()
• read_csv()
• read_parquet()
• ...

100% of user-facing Ibis APIs should be covered.

This may involve some script/utility to check coverage and systematically plug any gaps. We may also manually go through this process once and do a better job going forward about ensuring new APIs are documented.

One obvious new section may be "Input/output APIs", with documentation on reading and writing from the file formats Ibis supports (CSV, Parquet, Delta Lake table, JSON, etc.)

This needs a bit more thought and input. Opening this issue for public input.