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 erroneous extension list directive from Project subheading
• Move Markdown content above jsonschema directives and examples

TO-DO:

• [ ] Render new lines in release description (I copied the relevant part of the description from the release schema instead).
• [x] 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) (PR created https://github.com/OpenDataServices/sphinxcontrib-opendataservices-jsonschema/pull/60)
• [x] Collapse objects that reference a subschema in jsonschema directives (e.g. Item/deliveryAddresses)
• [ ] Consider moving more normative Markdown content to subschema descriptions
• [ ] Check for additional information in reference descriptions, e.g. contracts vs Contract.

From https://github.com/open-contracting/standard/issues/1467:

• [ ] Move jsonschema directives and extension lists to the end of each section (in that order)
• [ ] Link to guidance using seealso (replacing "For more information" sentences and note admonitions)

[duncandewhurst]

* [ ]  ~Render new lines in release description~ (I copied the relevant part of the description from the release schema instead).

Noting that we should check that the definitions of contracting and planning processes from the description of the release schema are featured prominently in the documentation.

Edit: I opened an issue: https://github.com/open-contracting/standard/issues/1714