Move MEPs into main documentation

I am proposing to move the MEPs from the wiki into the main documentation.

The main benifits to this are to make preserve the MEPs long term, to keep a better history of their development, and to improve the discussion around them.

The motivation behind using the wiki was to reduce the barriers to working on the MEP, but it has not worked out as well as hoped. Having to modify MEPs via a PR does increase the barriers on working on them, but it allows us to apply the tools we already use for code review to them.