[Leitwolf11]

Point the Progmasoft-authored code at Progmasoft C++ Style

The contributing guide described only the style of the code derived
from Catch2. The code under src/Progmasoft/ and tests/Progmasoft/ is
not derived from it and follows Progmasoft C++ Style
(https://github.com/Progmasoft/progmasoft-cpp), the C++ guide of the
other Progmasoft projects.

A new section under "Writing code" says so, keeps the upstream style
for the Catch2-derived code so upstream changes still apply cleanly,
and records the two deviations the Catch2 API requires: the namespace
Progmasoft::Catch3, and exceptions, which report a failed assertion and
stay inside the library.

[Leitwolf11]

Self-review from the diff: docs/contributing.md only, 16 added lines in a new section under 'Writing code'. It states that src/Progmasoft/ and tests/Progmasoft/ follow Progmasoft C++ Style while the Catch2-derived code keeps the upstream style described on the page, and records the two deviations the Catch2 API requires: the Progmasoft::Catch3 namespace (checked against the sources) and exceptions that stay inside the library. It makes no claim about CATCH_MOVE, since the Progmasoft code uses std::move. 88 checks pass.