Update the Readme with better copy and details on how to find us on Discord

[byjlw]

removed slack, and dev discuss references
Added Discord links
Better header and organization
Cleaned up the copy and removed duplicate copy

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/8126

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[github-actions]

This PR needs a release notes: label

If your changes are user facing and intended to be a part of release notes, please use a label starting with release notes:.

If not, please add the topic: not user facing label.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "topic: not user facing"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.

[guangy10]

BTW, our documentation side (https://pytorch.org/executorch/main/index.html) is referencing a different et-logo. Let me file a issue for it

[guangy10]

FYI, #8163