docs: refine README.md

[MichaelCurrin]

I cleaned up the markdown to make the code and result more readable.

Note that --watch is implied can be out. I added --trace as it helps for debugging errors verbosely.

Having a warning for needing Ruby seems out of place, so rather listed Ruby as dependency via a badge and in install section.

I added a Jekyll badge which shows 3.9 because this works on GH Pages. if your theme works on Jekyll 4, then it can be be 3.9, 4.X rather.

Also added a missing bundle config line - if you omit that, then bundle thinks it is running for the user level and not the repo level.