Add new-style readme build badges, bump actions/checkout in README.md examples

[magnetikonline]

Some quality of life things in README.md:

• Using the "new style" build/workflow buttons, which also links better to the action run logs under the actions tab.
• Added TOC to README.md generated with (self plug!) https://magnetikonline.github.io/markdown-toc-generate/
• Some whitespace clean up.

[jpogran]

Thank you for the help with the documentation. This looks great with the exception of the TOC. Your project looks very useful in other contexts, but HashiCorp follows a general format for a README and we don't include a TOC in most cases. This README isn't long enough to warrant one and GitHub already creates anchors for all of the headers to link to.

[magnetikonline]

No dramas @jpogran - have rebased and removed be5405d.