Clarify source build docs for qt6 branch and cmake compat#124

trasnake87 wants to merge 2 commits intominecraft-linux:mainminecraft-linux/minecraft-linux.github.io:mainfrom

trasnake87:fix/source-build-branch-instructionstrasnake87/minecraft-linux.github.io:fix/source-build-branch-instructionsCopy head branch name to clipboard