[roger-zhangg]

Which issue(s) does this change fix?

N/A

Why is this change necessary?

Several README links are broken or redirect to moved pages, and the Slack invite link is no longer valid.

How does it address the issue?

• Updates links to their current locations: install guide, Hello World guide, sam sync docs, AWS Toolkit page, Powertools for AWS Lambda, roadmap wiki page, version badge and developer guide.
• Replaces the retired CI/CD workshop with the SAM pipelines guide.
• Replaces the archived developer guide repository with the docs feedback link.
• Removes the Slack channel links.

What side effects does this change have?

None, README only.

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Review the generative AI contribution guidelines
• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.