Add subschema descriptions

[duncandewhurst]

Closes #1700

• Add subschema descriptions
• Remove Markdown content that duplicates information contained in a subschema description
• Add introductory sentence before jsonschema directives
• Remove erroneousextension list directive from Project subheading
• Move Markdown content above jsonschema directives and examples

TO-DO:

• Render new lines in release description
• Check why some examples show parent object and some don't (e.g. ImmediateContainer)
• Follow up on issue with the include option in the jsonschema directive (identifier is rendered when only id and name are specified in OrganizationReference)
• Collapse objects that reference a subschema in jsonschema directives (e.g. Item/deliveryAddresses)
• Consider moving more normative Markdown content to subschema descriptions