[13.x] Added docs for valet reading composer.json

[indykoning]

Docs for: laravel/valet#1554

This PR adds support to fall back to the php version defined in the composer.json
Due to using composer/semver all combination of php requirements are possible (though the match is not exact as we only care about the x.x version, no patch versions)

Requirements like ^8.3, 8.3, <8.5, >=8.4 are all supported.
Priority is still given to the valetrc files, and we pick the lowest matching version so ^8.3 could not suddenly switch to 8.6 once it comes out.
This is the same way rectorphp/rector handles php constraints.