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[bot]]

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

[knative-prow[bot]]

Hi @garg-dhruv. Thanks for your PR.

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[dsimansk]

Thanks for the PR! Welcome to the Knative community!

/ok-to-test

[codecov[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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](https://codecov.io/gh/knative/client/pull/1799/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=knative) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=knative). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=knative)

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[dsimansk]

/retest

[dsimansk]

/approve /lgtm

[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

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

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