gmail: enhanced documentation

[decarteret]

Summary

Enhanced documentation in README.md

Fixes #

Details

Old documentation demonstrated access token retrieval using 3rd party application Postman.
New documentation is demonstrating access token retrieval using Google Developers OAuth playground which is easier and more intuitive.

AI Usage

Please disclose how you've used AI in this work (it's cool, we just want to
know!):

• Code generation (copilot but not intellisense)
• Learning or fact checking
• Strategy / design
• Optimisation / refactoring
• Translation / spellchecking / doc gen
• Other
• I have not used AI

You can read more details in our
Responsible AI Policy

Review Checklist

Before merging, the reviewer should check the following items:

• Does the PR do what it claims to do?
• If this is a new adaptor, added the adaptor on marketing website ?
• If this PR includes breaking changes, do we need to update any jobs in
  production? Is it safe to release?
• Are there any unit tests?
• Is there a changeset associated with this PR? Should there be? Note that
  dev only changes don't need a changeset.
• Have you ticked a box under AI Usage?