Content type structure and navigation

[rmccreath]

Is your feature request related to a problem? Please describe.
Content categorisation is difficult to navigate leaving users unable to find relevant materials. In particular, textbooks are shown in a separate section at the bottom of the 'develop' pages but it's not clear to users:
Public-Health-Scotland/technical-docs#97

Describe the solution you'd like
Redesign the structure of the application to ensure navigation for specific topic and categories of content is easier to find. The first thought would be to split out long-form (e.g. textbook) content in a different page.