The testing guide stated OrcaSlicer uses Catch2 v2 and advised the v2
`<catch2/catch.hpp>` include, but the vendored framework is v3.11.0
(tests/catch2/) and every test file includes `<catch2/catch_all.hpp>`.
The wrong version drove several incorrect claims: that SKIP() is
unavailable (it is, v3.3.0+), that the string matcher is "Contains"
rather than "ContainsSubstring", and that thread-safe assertions,
multiple reporters, STATIC_CHECK and built-in sharding do not exist.
Correct all version statements, the example include, and the
former "Version-Specific Limitations" section to reflect v3.11.0.
454335dba6