DOC - Update documentation

[trallard]

Fixes https://github.com/conda-incubator/conda-store/issues/788

Description

This PR adds the following improvements to docs:

• Update intro and contribution guides
• Adds og:image for social which was missing

Pull request checklist

• [x] Did you test this change locally?
• [x] Did you update the documentation (if required)?
• [ ] Did you add/update relevant tests for this change (if required)?

Additional information

How to test

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