docs: Add config instructions for clangd with libvirt

In build environments which use gcc as the default compiler, use of the
clangd LSP server (for enhanced code editing and navigation etc.) with
libvirt requires some additional configuration.  Detail this and link
from `hacking.rst`.

Signed-off-by: Tim Small <tim@seoss.co.uk>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

docs/hacking.rst

@@ -28,6 +28,9 @@ The `"Git checkout" section <compiling.html#git-checkout>`__
 of the libvirt installation instructions give an overview of the
 compilation process.
 
+Optionally, `Clangd with libvirt <clangd.html>`__ can be used to
+navigate the code base etc. within most code editors (and IDEs).
+
 Preparing patches
 =================