Find_SDL*: Update documentation

This updates the following modules:
- FindSDL
- FindSDL_gfx
- FindSDL_image
- FindSDL_mixer
- FindSDL_net
- FindSDL_sound
- FindSDL_ttf

Changes:
- Synced modules documentation.
- Added examples sections.
- Used the ``<PackageName>_FOUND`` variables. The uppercased
  ``<PACKAGENAME>_FOUND`` variables are also set to the same value as of
  CMake 3.3.
- Added notes regarding the CMake package configuration files
  provided by upstream distributions with separate imported targets.
- Find modules index sorted alphabetically for FindSDL* modules.