Reorganize the pages of the documentation

The index page should be reorganized. They need to cover as much of the different areas of the Qt API as possible without overlapping too much.

All page elements will be replaced by more generic elements, and all elements containing doc content will receive an ID, making them searchable and more flexible to other features.

There will be three types of pages in the docs:

• Overviews (including the index page)
• Reference pages (Module/Class reference etc.)
• Article pages (In-depth articles etc.)

All three will share the same header, footer and navigation menu.