cmake: Use "coverage" preset instead of "Coverage" build type and document it

[hebasto]

Using -DCMAKE_BUILD_TYPE=Coverage to configure the build for coverage analysis has a flaw as build types are not defined at the configuration stage for multi-config generators.

This PR implements the idea suggested here.

Please note that both added to the README.md commands, cmake --build build and ctest --test-dir build, accept the -j $(nproc) option to parallelize work.

Drafted as it is based on #1234.

[real-or-random]

https://clang.llvm.org/docs/SourceBasedCodeCoverage.html clang supports this too, so no need to set a default here, I think.

[hebasto]

Doesn't clang require the use of the llvm-cov tool, which is currently undocumented in the README.md?

[theuni]

Concept ACK assuming one can stack multiple presets at once.

[hebasto]

Rebased.

[theuni]

After a discussion about this in-person, we've found drawbacks in the build type approach as well as the preset one.

Instead, let's just do the simple thing and hook up a new option.

[hebasto]

After a discussion about this in-person, we've found drawbacks in the build type approach as well as the preset one.

Instead, let's just do the simple thing and hook up a new option.

Closing in favour of #1291.