docs: Add a section to HACKING.md on using mkosi and clangd together

docs: Add a section to HACKING.md on using mkosi and clangd together

While it's perfectly possible today to completely rely on mkosi for
building and testing systemd, to get code completion and other IDE
niceties to work properly, it's still necessary to build systemd
locally.

Recently, mkosi gained the ability to allow external programs to
communicate with the build script. We can use this feature to run
the clangd language server in the mkosi build image via a custom
build script to provide IDE features in editors without requiring
developers to build systemd on the host or install any of systemd's
build dependencies locally.

This commit adds the necessary information on how to set this up
to HACKING.md.