3499: Improve style consistency throughout docs.

* Capitalize Dashboard.

* Article use: "access Workbench," "access the Dashboard."

* Bold references to Workbench UI elements.

* Make example Arvados environment variables more realistic.
3499: Update pipeline tutorial for recent UI changes.
3499: Update Keep tutorial for recent UI changes.
Merge branch '3499-home-project-docs-wip'

Closes #3516.  Refs #3499.