Add a help page for CTF available through command /ctf_help

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The markdown2formspec mod is required for this to work.

This is a bit similar to the "Help" inventory tab from CTF v2.

A HUD that prompts players to view the help page will be shown to players joining the game for the first time.

The instructions are left short and vague on purpose so that players are put on the right path but still have the opportunity to explore and discover the game.