WIP: rewrite relative links to additional pages

[Gankra]

this is currently hardcoded to SECURITY.md, and needs a more robust notion of relative links to pages we care about

(also this temporarily adjusts the repo to give a test case)

[ashleygwilliams]

from @Gankra:

this PR needs much more work, but i think a lot of that work will happen while we work on monorepo (#493) stuff
the missing piece is a mapping from page ~names to page ~paths, so that the link rewriter knows what to rewrite.

that PR is basically providing the logic for editing the results of the markdown parser before rendering to html which I wanted to confirm was doable. ideally before we start rendering pages we have a listing of every page we will render so relative linking can work well (and maybe this also reduces the amount of need we have for path-prefix, if every page knows where exactly all other pages are in the hierarchy?)