Update documentation overview page
The existing overview page for GIO is quite dated and has not been updated to include new APIs for a while now.
In order to provide a nice route into the documentation for people who prefer to browse rather than to search, it would be good to update this and make it a bit easier to visually navigate. All the GIO classes should be listed, but grouped by their area of functionality (for example: networking, file access, resource monitoring), with some brief explanation of what each group is.
One idea would be to make it look a bit like a combination of these two pages:
- https://developer.gnome.org/documentation/introduction/overview/services.html, because it explains each group of classes briefly but non-trivially
- https://developer.gnome.org/hig/guidelines.html, because it has nice icons which allow easily visually identifying a topic/group of APIs which GIO provides