Update and reorganise kn cli setup docs

[garg-dhruv]

Description

Resolves inconsistency in install instructions between docs/README.md and the official website.
Resolution: Point user directly to official website and delete docs/README.md

Also, moving docs/DEVELOPMENT.md to root directory in alignment with other Knative projects.

Reference

Fixes #1410

[knative-prow]

Welcome @garg-dhruv! It looks like this is your first PR to knative/client 🎉

[knative-prow]

Hi @garg-dhruv. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[dsimansk]

Thanks for the PR! Welcome to the Knative community!

/ok-to-test

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.03 ⚠️

Comparison is base (3a8d646) 79.87% compared to head (23685e8) 79.85%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1799      +/-   ##
==========================================
- Coverage   79.87%   79.85%   -0.03%     
==========================================
  Files         174      175       +1     
  Lines       13593    13633      +40     
==========================================
+ Hits        10858    10886      +28     
- Misses       1992     2000       +8     
- Partials      743      747       +4     

see 3 files with indirect coverage changes

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[dsimansk]

/retest

[dsimansk]

/approve
/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dsimansk, garg-dhruv

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