docs: adds a json schema namespace

[baywet]

follow up to #85, #86, #88, #87, #84, #89, #97, #96, #95, #94, #93, #92, #91, #90

[handrews]

Can we make a note that, strictly speaking, x- prefixes are only required in 3.0 or earlier? For 3.1+, OAD authors should check their tool documentation to find out whether they support x- schema keywords or some other mechanism for accessing keywords from later drafts (which, IIRC, some JSON Schema implementations do, at least for some keywords).

[mikekistler]

@handrews are you suggesting that we also reserve the jsonschema namespace (with no "x-")? I suppose we could do that, but I think that would mean that extensions in that namespace could only be used in OpenAPI 3.1 or later, and I don't think any of the extensions currently proposed or approved fall in to this category.

[handrews]

@mikekistler no, I mean tht some JSON Schema implementations will just allow, say, propertyNames with draft-04 even though it wasn't introduced until later.

Although come to think of it, this wouldn't be a problem in 3.1+ unless you had already set jsonSchemaDialect to something like draft-04, and if you wanted to use later keywords you wouldn't do that, so... yeah, never mind.

I see some extension entries say something like "used when targeting OpenAPI versions prior to 3.2", so I think this whole namespace should have "used when targeting OpenAPI versions prior to 3.1" on it, and that would be sufficient. Ignore all the rest of what I said.

[baywet]

Suggested change

	The `x-{{page.slug}}-` prefix is reserved for OpenAPI Specification Extensions that mirror JSON Schema keywords when targeting OpenAPI versions or JSON Schema dialects where those keywords are not directly available.
	Extensions in this namespace are registered in the [OpenAPI Extension Registry](../extension/), like the `x-oai-` and `x-oas-` namespaces.
	{% endcapture %}
	The `x-{{page.slug}}-` prefix is reserved for OpenAPI Specification Extensions that mirror JSON Schema keywords when targeting OpenAPI versions prior to 3.1 or JSON Schema dialects where those keywords are not directly available.
	Extensions in this namespace are registered in the [OpenAPI Extension Registry](../extension/), like the `x-oai-` and `x-oas-` namespaces.
	{% endcapture %}

does that work?

[mikekistler]

Looks good. 👍