Added documentation to use the Discogs backend

[carlio]

Proposed changes

This is the documentation for adding the Discogs OAuth1 backend.

Types of changes

• [ ] Release (new release request)
• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Code style update (PEP8, lint, formatting, renaming, etc)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Build related changes (build process, tests runner, etc)
• [ ] Other (please describe):

Checklist

• [x] Lint and unit tests pass locally with my changes
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added documentation to https://github.com/python-social-auth/social-docs

Other information

The PR to add the functionality is https://github.com/python-social-auth/social-core/pull/853

[nijel]

Looks okay rendered: https://python-social-auth--187.org.readthedocs.build/en/187/backends/discogs.html

[nijel]

Merged, thanks for your contribution!