Change 3.latest reference links to 3.stable

3.latest should only be used when linking to something that's still in RC; once
a final release with that feature goes out, it should change to stable.
PRM: Add list of pending "directory" pages to sidebar

We're about to add these pages one by one.
PRM: Add page on ssldir
Change wording of location section for config files
PRM: Add hiera.yaml to config file list

These things multiply like little configurable bunnies.