docs: Added OpenStreetMap OAuth2 Documentation

[mmd-osm]

Proposed changes

Added docs for OpenStreetMap OAuth 2.0.
Also added deprecation notice for OAuth 1.0a, according to official communication in https://community.openstreetmap.org/t/oauth-1-0a-and-http-basic-auth-shutdown/108490

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python-social-auth/social-core#877

[nijel]

Merged, thanks for your contribution!