* Update TestData docs

Fixes TestData docs by correcting name, installation instructions, etc. Should prevent issues like #16386.

- Fix name: `Grafana TestData` -> `TestData DB`
- Fix "Enable" section instructions
- Fix "Dashboard" section instructions
- Fix grammar

* Bold clickable items, list step-by-step instrs.

* Apply suggestions from code review

Co-Authored-By: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>