Development (schema): Update extended JSON Schema section

[duncandewhurst]

os4d/docs/development/schema.md

Lines 47 to 58 in e64cc74

	## Extended JSON schema
	We use a number of custom properties in our JSON Schema implementation. A [patch against JSON Schema 0.4 to include these is found here](https://github.com/open-contracting/standard/blob/6e538252dd08344222b5cd16b864ed0a2a866197/standard/schema/metaschema/meta-schema-patch.json).
	```eval_rst
	.. todo::
	We need to document the fields in our schema patch.
	We should also explore moving our JSON schema meta-schema patch to our own opendataservices repository and managing it there.
	```

[duncandewhurst]

We need to document the fields in our schema patch.

The fields are documented in this PR: open-contracting/standard#1723

We should also explore moving our JSON schema meta-schema patch to our own opendataservices repository and managing it there.

This sounds like a good idea. If we do it, we should remove the OCDS-specific fields (omitWhenMerged and wholeListMerge), remove the reference to OCDS from deprecation.deprecatedVersion, and rename deprecated to deprecatedDetails so that the patch is compatible with both JSON Schema draft 4 and 2020-12, as discussed in Planio-53514.

Once in our own repository, we can add a jsonschema directive to display the fields.

[duncandewhurst]

Metaschema patch repository created: https://github.com/OpenDataServices/json-schema-extension with the updates detailed above. It's private while I sort a few things out.