Refresh README

[jonathan343]

Note

This PR builds on top of and conflicts with #2162 and #2163 so it should be merged after. This diff will minimize after they are merged.

Addresses #2151 and #2142

Summary

Quick refresh of README.rst:

• Badges: drop dead Gitter badge; add PyPI version, supported Python versions, and license badges
• Logo: fix broken logo image link
• Links: switch to HTTPS for shields.io badges, the docs link, and the readthedocs target; update the boto3 configuration docs link to its current home on docs.aws.amazon.com
• Examples: normalize illustrative code snippets to double quotes and f-strings; keep the "Let's take a look at the app.py file" block on single quotes so it matches what chalice new-project actually writes
• Walkthrough fixes: repair malformed ARN in the chalice delete example, use a 12-digit placeholder account ID, align the deploy-output stage name with the rest of the doc (/api), and reuse the abcd... placeholder host instead of a hardcoded one

Testing

• Render README.rst on GitHub and confirm badges, logo, and external links resolve

---

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