For the 0.17.0 changelog - add a "what's new in 0.17.0" page to the main ReST docs

[Dunbaratu]

To keep the changelog from being too verbose, have it just link to more long descriptions over in the main ReST docs. - I think we should have a single page for just "what's new" for the existing users to pore over so they don't have to read the entire document tree again.

The page would occupy the halfway point between the terseness of the changelog (which would JUST mention things like "user functions" as a one-liner, and link to this page), and the verbosity of the full rest of the documentation.

The intended audience would be people who already know kOS and just need to find out what changed, and be given a bunch of links into the places in the main docs where the changes are fully described.

[Dunbaratu]

Moved this to just a comment under issue #716 , to reduce the amount of extra issue numbers.