docs: Add basic OAuth client docs

[tennox]

As discussed here: #875 (comment)

Issues: #875 and #1031

[codecov]

Codecov Report

Merging #1080 (c3b3d47) into master (3f02186) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1080   +/-   ##
=======================================
  Coverage   95.24%   95.24%           
=======================================
  Files          81       81           
  Lines        1830     1830           
  Branches      408      408           
=======================================
  Hits         1743     1743           
  Misses         85       85           
  Partials        2        2           

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[agustif]

Hey @tennox would be cool if you can make a separate nextcloud package for easy imports, at least I'm gonna do that when I can updating the Insta one and adding the FB one I've working for my project . But awesome start, thank you for contributing!!

[pradel]

@tennox thank you so much for the pr that's amazing 🚀