docs: updates oauth docs with rfc-6819 examples

[losandes]

Summary

Updates the examples and documentation to implmenent RFC-6819 Section
5.3.5: Link the "state" Parameter to User Agent Session. These examples demonstrate how to mitigate Cross Site Request Forgery (CSRF) by synchronizing all 3 parties (user, Slack, app) in the OAuth flow. Uses the synchronizer token pattern (STP) and a JWT embedded in a cookie to bind the token to the device.

NOTE: this is an alternate to #1013 that introduces no changes to the library; only to the examples

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #1014 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1014   +/-   ##
=======================================
  Coverage   94.40%   94.40%           
=======================================
  Files          12       12           
  Lines         768      768           
  Branches      173      173           
=======================================
  Hits          725      725           
  Misses         14       14           
  Partials       29       29           

Flag	Coverage Δ
#eventsapi	89.61% <ø> (ø)
#interactivemessages	95.04% <ø> (ø)
#webapi	96.36% <ø> (ø)
#webhook	87.50% <ø> (ø)

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