Split the Libraries page as UI Libraries, Core Libraries
Fixing #17. This commit splits the Libraries page of the Platform Components section into two separate pages: User Interface Libraries, and Core Libraries.
Some aesthetic changes were made to the markup in the process; each section that pertains to a specific project within the GNOME platform now links to a central home-page for the project (else the README on GitLab).
Section hierarchies have been flattened/rearranged, so that each of these page's subsections is a category that contains projects. In the previous version, there were a minority of subsections that existed to describe only one library. I feel this change brings some consistency to how these pages are organised.
Some phrasing was changed slightly to reflect this new structure, and to ensure that each "category" (e.g. "Fonts & Rendering", ...) has some form of introduction text.