merge HACKING with CodingStyle.txt into HACKING.md
HACKING: Merge with CodingStyle.txt
HACKING references CodingStyle.txt, both have different structure.
Having two files can confuse developers.
Merge the information, make extra headings for the Vala Code and C Code parts of the Coding Style Guidelines. Remove unnecessary UI Guidelines.
Fixes: https://gitlab.gnome.org/GNOME/gnome-games/-/issues/281
Edited by siroj42