Add API documentation

[nasainsbury]

API documentation is needed for Octopus, we're likely want this as html and not just markdown.

Will investigate good ways to host and document the api documentation, hopefully so that it "blends in"/"feels like" the rest of the site.

[ghost]

Possibly linked to Jira ticket 133

Assigning to @nathan-at-jisc until future assignee

[create-issue-branch]

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