Guide on modoptions and document default engine modoptions

A guide on how to pass and use modoptions on games.

Note the engine [hardcodes a few keys for modoptions with defaults](https://github.com/beyond-all-reason/RecoilEngine/blob/5e0e29257671e389e2e1e375f45bdb45f9d206c5/rts/Game/GameSetup.cpp#L628-L638), for hardcoded behavior as well. We must have these documented.

This task is related to: https://github.com/beyond-all-reason/RecoilEngine/issues/2458. Notice we shouldn't need to provide much detail on how to pass modoptions via startscript, since the startscript guide should provide for it.

Read rts/Game/GameSetup.cpp at lines 628-638 to identify the hardcoded modoption keys and defaults, then review the related startscript guide and issue #2458. Done means a guide explains how games pass and use modoptions and documents the engine defaults without duplicating startscript details.

Written by the indexing model from the issue text.