Usually we point to the website. But honestly it's cumbersome for
developers to update the website. Let's decide later whether we remove
the section there or how we keep it in sync.
But for now let's just add a detailed build section here.
Especially since now we ship autotools *and* meson.
Also mention the convention that we use `build_run` as the meson build
dir. The quality script will depend on this.
Update the gh workflow to pin clang-format to version 21.
Mention the version we use in CONTRIBUTING.md.
And hint to the workflow file if we want to change it.
Move the `make doublecheck` functionality into a build system agnostic
script.
`scripts/quality-check.sh` can now be used to check for spelling via
codespell, formatting clang-format and run the unit tests.
`make doublecheck` and `meson compile doublecheck` will call this
script.
Sometimes we have issues with different versions of clang-format locally
vs our CI. In this case SKIP_FORMAT env variable can be set.
We need to define some rules to make sure people know what they need to
take care about.
To allow us to later still understand the changes in an optimal way and
make everyones work easier.
