Bring the page a lot closer to GitLab tone and style. Not perfect,
but we've gotta start somewhere.

It helps to have less verbose README.md

These pages should get credited to Source Code. (I should also
check about CODEOWNERS entries too.)

Do an initial cleanup pass:

- Line wraps
- Change 'allows' to 'enables'
- Clean up syntax highlighting
- Fix spelling and capitalization
- Standardize on colons at the end of the introductory sentences

Describe what Gitlab Shell is capable of