docs: fix broken intersphinx hyperlink

The intersphinx extension was producing the wrong href for the link from
whats-new/index to the design proposals. Replaced with a hardcoded
hyperlink for now.

https://bitbucket.org/birkenfeld/sphinx/issue/1204/

Change-Id: I09d87499e4831c8168dccfb315fc14e77554e3a4