Add candidate initiatives and use searchfilter plugin

[choldgraf]

This does two things:

1. Adds candidate initiatives and the logic for them described here: Add "Candidate Initiatives" to our public roadmap infrastructure#7859
2. Uses the myst-plugins searchfilter, since that's a self-contained little plugin that means we don't have to duplicate here

[choldgraf]

I'm gonna try merging this in to see if it works in gh pages