[DOC] Add JupyterLab Extension docs page

[pavithraes]

Addresses #561

Description

This pull request:

• Creates a landing page for JLab extension with installation instructions.

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)? N/A

Additional information

How to test

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[pavithraes]

@gabalafou - Thanks for a thorough review, I've addressed all your comments. :)