Rework README.md

[t-mart]

This PR reworks the README.md file. I just used this template for my own bucket, and saw some areas that could be improved:

• Reorder the sections, putting the "How do I use this template?" first. This is the first way you'll interact with this template.
• Provide step-by-step instructions to using the template, elaborating in certain areas. Specifically, instruct how to set the GitHub Actions write permission (see [Bug]: Excavator action fails due to permisisons #8).
• Show app installation commands in a way similar to how https://scoop.sh does.
• Reference the App Manifests wiki page too, which includes detail about manifest keys.
• Use consistent markdown heading syntax (get rid of the underline style).

Closes #8

• I have read the Contributing Guide.