DOC - Update documentation

[trallard]

Fixes #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

• Did you test this change locally?
• 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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[pavithraes]

Thanks, Tania. This looks good to me!

We can eagerly merge this, and I'll incorporate your updates in #763 which restructures the information a bit.

I'll re-trigger the CI and Netlify builds, because they're red here but PR had no issues locally.