[FEATURE] Add Right Click to Shortcut documentation

**Is your feature request related to a problem? Please describe.**
It was hard to figure out how to pan across a large diagrame (>100 tables). Found out from the Issue tracker that right clicking toggles a grab and pan mouse mode.

**Describe the solution you'd like**
Please note in the Shortcuts (linked from the app) documentation that Right clicking allows paning the diagram.

**Additional context**
Would note that on Chrome, right clicking initally opens a popup menu, and the pan mode doesn't always activate.

Open the Shortcuts documentation linked from the app and find where mouse controls are described. Document that right-click toggles grab-and-pan mode, including the Chrome context-menu caveat. Done means the shortcut documentation explains how to pan large diagrams and notes the activation issue.

Written by the indexing model from the issue text.