[STAR-2754] Fix links for Value Proposition Template and Client Briefing Template
Description
This fixes links in the repo to template files.
MkDocs does not render Markdown files which aren't in the navigation. Rather than attempt to fix this bug, we're elevating it to 'feature' since we don't want to use the same rendering theme that the rest of the handbook does, and GitLab already provides one. So, included are updated links and an updated way to build new links in case other files in this repository need to be used in a similar way.
Testing instructions
Use the 'view app' button on this MR, and browse the tree to verify the new README runs correctly. Also you can build locally (after initial setup, see the repo README) with make run
and test there.