[2021.2] Backport of PR #5599 - GSG-546 & GSG-547 Updates to README/Index.md

[sarah-welton]

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Checklist for PR maker

• Have you added a backport label (if needed)? For example, the need-backport-* label. After you backport the PR, the label changes to backported-*.
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Purpose of this PR

Why is this PR needed, what hard problem is it solving/fixing?

Made changes to About Shader Graph page and README file to satisfy changes needed by GSG-546 & GSG-547

"Backports" are needed for the changes made to the README file (with some modifications) for 7.x.x, 10.x.x, and 2021.1.

"Backport" needed for the changes made to index.md for 2021.1

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Testing status

Describe what manual/automated tests were performed for this PR

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Comments to reviewers

Notes for the reviewers you have assigned.

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
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Search for your PR branch using the sidebar on the left, then add the following segment(s) to the end of the URL (you may need multiple tabs depending on how many packages you change)

Shader Graph
/.yamato%252Fall-shadergraph.yml%2523PR_ShaderGraph_2021.2
Depending on your PR, you may also want
/.yamato%252Fall-shadergraph_builtin_foundation.yml%2523PR_ShaderGraph_BuiltIn_Foundation_2021.2
/.yamato%252Fall-shadergraph_builtin_lighting.yml%2523PR_ShaderGraph_BuiltIn_Lighting_2021.2

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