Document: Multiple locations, shapes, and the new browse map

[jimsafley]

The next release changes most of what the document page describes. That page is written for one location per item, so this is a rewrite rather than an addition, and the screenshots will need retaking.

What is new:

• Multiple locations per item, and polygon, polyline and rectangle shapes alongside points
• A "fit all" control on every map
• The browse map's "show item list" option, and its list grouped by item
• Sequence mode in the ExhibitBuilder block (Sequence mode #70)
• Basemap changes: the default is OpenStreetMap now, with API key settings for Carto and Stadia (Testing: Basemap fixes and provider credentials #82, fix/basemap-providers branch, testing not finished)