Add tutorial on how to build software manually

This MR:

• mentions how to build software using only CMake commands
• makes opinionated recommendations (Ninja on Linux, MSVC on Windows, use only CMake commands, use --parallel and --prefix, prefer build directory inside project source)
• mentions Windows instructions

Why?

• this can be later linked to from other tutorials like KXmlGui, Kirigami, kdesrc-build, Craft and distrobox
• we actually do have KDE developers that do not use kdesrc-build or Craft
• we need a good reference to point new devs to whenever they are in doubt about how to build a project

This MR does not:

• include instructions on building KDE software on Windows without Craft (yes, Craft is that good)
• intend to be a tutorial strictly speaking, but a task-based reference guide (it focuses on how to do what the user likely wants, but does not handhold the developer)

This has been tested on Plasma 6, on a Fedora 40 distrobox on the Steam Deck, and on Windows 11.