Add examples of JSON to be sent to the API

These are based on what we think we need to send to the content-store.
Some fields, such as a section's manual slug, will be used to generate
base_path and routes and will not be included in the data sent to the
content-store.

The content of these examples is derived from the scraped JSON in
manuals-frontend. We have simplified it to make it shorter and easier
to read, and the relationships between sections and the manual are not
consistent (eg the sections in EIM25525's breadcrumbs are not included
here).
Add JSON schemas for data to be sent to the API

We haven't yet decided exactly how optional fields should be handled;
we want consistent data in the content-store for fields which aren't
present, and some ways of achieving that may involve adding restrictions
for those fields in the schema, but we think that the structure of the
fields themselves is unlikely to change significantly.

This also includes validation tests of minimal and maximal JSON examples,
which are easier to read than the schemas themselves.
Validate JSON request examples against the schemas

These examples were converted by hand from the scraped JSON in
manuals-frontend, and as the schemas evolve they are very likely to
become invalid JSON and inconsistent with the schemas. Validating
the examples against the schemas ensures that this doesn't happen.

This commit also removes some invalid trailing commas and fixes the
manual slug in EIM25525.