Document mouse-button panning in the shortcuts reference

[NickYCLin]

The shortcuts page omits mouse-button panning, making it difficult to discover how to navigate large diagrams. Add right-button and middle-button dragging to the canvas controls table, and explain that panning lasts while the button is held rather than toggling on a right-click.

Addresses drawdb-io/drawdb#1107. This matches the existing Canvas navigation section in customizing-editor.mdx and the editor's pointer handlers; no editor behavior changes.

Validation:

• npm run check (route types, TypeScript, and ESLint).
• npm run build with the GitHub Pages /docs base path.
• Chromium: verified right- and middle-button dragging change the canvas view, releasing stops panning, and a right-click followed by pointer movement does not pan.
• Checked the rendered shortcuts page at desktop and mobile widths.

Preview limitation: the Windows static preview emitted route-prefetch 404s. The directly loaded shortcuts page rendered correctly at both widths; this check does not establish that all client-side navigation works.