Filter Ext: Provide more detailed STAC-specific queryables recommendations

[philvarner]

The document makes an arbitrary distinction between "regular" queryables and "synthetic" queryables. All queryables are synthetic, so this isn't a true distinction.

For STAC, there should be better recommendation about what a queryables schema should be, and advice on how common STAC extensions should expose their fields as queryables. Canonical example is "traversing" lists and nested objects, e.g., eo:bands common_name.