Editorial: Add a Feature Matrix as summary

[azaroth42]

... with links to the sections of the different specs as to where the features are specified.

Features should be atomic, if possible, and cover things like paging rather than just the headings currently in the doc. Thus the rows would be features, the columns would be the specifications, and the cells be links to that feature in that spec, or empty. This would allow a nice overview, and demonstrate areas that are converging while allowing easy access to the specs where the details differ.

[rhiaro]

+1. PRs welcome if I don't get there first

[rhiaro]

Feature matrix is there, thought still perhaps not as detailed as you'd like. I'll leave this issue open until we're happy with it.