doc: Declare CoglIndices
doc: Declare CoglIndices

Types need to be declare to be cross-referenceable. Use the opportunity
to reorganise the symbols in subsections.

gcampax/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl,djdeath/cogl-android,djdeath/cogl,djdeath/cogl,gcampax/cogl,djdeath/cogl-android,djdeath/cogl-android