[DOC] Update contributing guidelines

[pavithraes]

Addresses #748

Description

This pull request:

• Separates setup instructions for conda-store, conda-store-ui, and jupyterlab extension
• Fork and clone steps are similar, so they are bundled together
• Updates docs contribution instructions

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

Squash merge this PR.

How to test

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[pavithraes]

TODO: Incorporate updates from #792

[gabalafou]

One follow-up task for this PR right after it is merged:

• Update the URL in the "Development" section of the README in the conda-store-ui repo.