docs(custom-claims): add documentation for custom claims class usage with ORCID example by MartinPaulEve · Pull Request #772 · authlib/authlib

This PR creates documentation for using the new custom-claims feature, as per #725

DO NOT SEND ANY SECURITY FIX HERE. Please read "Security Reporting" section
on README.

What kind of change does this PR introduce? (check at least one)

  • Bugfix
  • Feature
  • Code style update
  • Refactor
  • Other, please describe: docs

Does this PR introduce a breaking change? (check one)

  • Yes
  • No

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