Avoid 'floating point or integer' distinction for JSON number values

[tomcrane]

Addresses #2492

There is a corresponding cookbook recipes PR: IIIF/cookbook-recipes#722

Generally, use "number" rather than "floating point number", and specify in Terminology section that "Where a value is required to be an integer, this is stated explicitly, and the value MUST be a number without a fractional part."

Use both forms in examples where valid to reinforce the point (e.g., 4 and 4.5)

Additional semantic change:

instant was "positive floating point number" but is now "non-negative number" because 0 is a valid instant.

@zimeon mentions "Consider typing in the @context" - e.g., adding "@type": "xsd:double" - but this would produce RDF changes in existing IIIF? And seems to go against #1291

This bit I am unsure of:

A number may have a fractional part, and may be written in any form permitted by JSON: for example, 1985, 1985.0 and 1.985E3 are all the same number.

It's correct, but we could for reasons of clarity, disallow the exponential form.

I don't think we should, because it might leak into Manifests inadvertently through serialisation, and is not invalid.