[docs/postprocessing] Improve Postprocessing documentation

[nartc]

Currently, angular-three-postprocessing documentation is barebone as we want to get v2 out and definitely can be improved.

• Add API documentation for the EffectComposer
• Add more examples and more gotchas (reference postprocessing and react-postprocessing)

[nartc]

This is on going in the docs repo