[DOCS] Add searchable API-Reference

I hope I haven't overlooked it, but could you please add an API reference page, where I can just search for e.g. a macro or function name and get forwarded to the documentation? This would be interesting for both discoverability (let's see what else/new things catch2 is offering) as well as for understanding existing code (What does SECTION mean?).

My specific use case was that I wanted to see what catch provides in terms of random number generation.

The issue names no files, tests, or entry points. Start by reviewing Catch2's existing documentation and public macros and functions, then define the searchable reference scope; done means users can search an API name such as SECTION or a random-number facility and reach its documentation.

Written by the indexing model from the issue text.