MMS (Multisig Messaging System) manual added to user guides

English manual for the MMS (Multisig Messaging System), available in the CLI wallet, as currently on course to become regular part of Monero with the Spring hardfork. (It was merged into the latest development code and is fully working as described in this manual, given that you compile from that source yourself).

The one deletion in this PR results from adding a line end at the end of file resources/user-guides/index.md which happened inadvertently, but I think it makes sense, so I left it.

The line width on user guide pages is quite small, with much unused whitespace left and right on those pages, which results now in many ugly line breaks in the sample CLI output contained in the manual that makes for difficult reading, but that's probably a problem for another PR, if correctable at all ...