Update documentation for v3

[horenmar]

Most of the documentation was written for v2, where there was a single
header to include, and thus is missing information which headers to include
(and sometimes proper configuration for the feature).

There is also a secondary project to move code examples in documentation
to be executable and thus CI tested via something like mdsnippets.

[horenmar]

Note on mdsnippets: they do not promise that hashes used in links are stable, thus potentially causing pointless churn in diffs. Using them also imposes extra work on contributors, requiring them to install a dotnet tool.

One way to avoid this issue is to only regenerate snippets as part of release procedure, alongside other automatic documentation updates, such as rewriting the version placeholders. When limited to once per release, the potential churn in diffs is reasonable, and it avoids imposing the requirement onto all contributors.

The downside of this approach is that breakage in documentation can be discovered much later than it happens.

[lucid-ben-sizer]

I would love this to include information about how to structure a project to make proper use of this framework. It builds perfectly using add_subdirectory and if I paste the factorial example into my code, that too builds fine. But I have no idea how to make or run a test suite. I can't find anything in the docs about the recommended directory layout or how a project would normally be configured to use catch2.