Avoid re-querying the list of pages
Avoid re-querying the list of pages

The list of pages now gets formed a constant number of times within a mkdocs build, instead of recreating the list for every page.

For compatibility (if someone happened to use this function externally), one can still pass a `Files` to get_context, but its main mode of operation is now to accept a direct list of `File`s
waylan/mkdocs,mkdocs/mkdocs,waylan/mkdocs,mkdocs/mkdocs,mkdocs/mkdocs,waylan/mkdocs