Fix building libnm documentation with meson

Summary

This merge request replaces the ignore_decorators RegEx with an equivalent one that does not use backslashes, and also builds docs for libnm-core-impl and libnm-client-impl, which were previously not included.

Purpose

Currently, the libnm documentation fails to build with meson due to meson replacing backslashes with slashes, and is also missing some parts of documentation when built with meson. This merge request introduces a workaround so that building it with meson works again, and is comparable to the documentation built by autotools.

Checklist

Please read https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/CONTRIBUTING.md before opening the merge request. In particular, check that:

• the subject for all commits is concise and explicative
• the message for all commits explains the reason for the change
• the source is properly formatted
• any relevant documentation is up to date
• you have added unit tests if applicable