Adjust README to show status badges from GitHub Actions workflows

[stmontgomery]

This adjusts README.md to begin showing CI status badges from the GitHub Actions workflows I configured in #1361.

View rendered README document.

Motivation:

This will help ensure our CI status badges remain consistent with the CI status shown on Pull Requests.

Modifications:

• Remove the CI status column on the "Supported platforms" table from the README.
  • This is because the new workflows do not offer granular, per-platform status badges (see GitHub's documentation).
• Add a new section to the README showing this status.
• Drive-by improvement: Move "Windows" platform up a few rows so that all the Supported platforms are listed first, followed by all Experimental platforms.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.