Xmake support

I was able to successfully use Catch2 `v2` with Xmake by simply adding `#define CATCH_CONFIG_MAIN` and `#include ` to the tests cpp file. However, I couldn't figure out how to make Catch2 `v3` work with Xmake yet. I get `undefined reference to 'main'`.

There's a page about [Cmake integration](https://github.com/catchorg/Catch2/blob/devel/docs/cmake-integration.md) but unfortunately there's no page about Xmake integration. It would be very helpful if such a file existed.

I'm guessing I have to link with `Catch2Main` but adding `add_ldflags("-lCatch2Main")` didn't help so I'm currently out of ideas.

Start with docs/cmake-integration.md and the reported Xmake setup for the tests cpp file. Investigate the Catch2 v3 undefined-reference-to-main problem and the attempted Catch2Main link, then document the working Xmake integration steps. Done means a clear Xmake integration page covers the v3 entry point and lets users build and run tests.

Written by the indexing model from the issue text.