Updated OAuth2 client documentation

All links for JavaDoc replaced with relative links to GH (JavaDoc was really obsolete).
Reordered chapters for more streamlined reading (resource -> client -> template -> token persisting).
Reworked all the resource details, client parameters and token persisting as was obsolote and misleading.

*NOTES:*
* Only modified the client section, if needed I can go with the provider but I am not familiarized with that part.
* Please someone proofread as English is not my main language.