docs: Make API document versioned

docs: Make API document versioned

OpenAPI doesn't appear to support versioning natively, suggesting
instead that separate documents are kept. Rather than doing this
manually, let's use a templating tool - Jinja2, in this case - to
generate these document for us from a single master document.

Note that while we can now auto-generate these whenever we need them
(and we tend to avoid storing auto-generated assets in VCS), these
change so rarely that it's easier to just store them. This also means we
can reference the schemas themselves online. We do this in a following
change.

Signed-off-by: Stephen Finucane <stephen@that.guru>