Add Gametypes to the guide

Add Gametypes to the guide

Descriptions were based on https://gitlab.com/xonotic/xonotic/-/wikis/home, although some gamemodes don't have a description there.
Added back .gametype_description (now called .m_description) so that the shorter gamemode descriptions can still be used.
Added .m_name to the gamemode class, copied with the initial value of .message, so that the COLORED_NAME macros can be used. Ideally .message wouldn't be necessary since it's not very descriptive, I just didn't bother going through all instances of .message being used to make sure nothing would break.