Adding request and response header documentation

Preliminary work for #4

 - Adds `requestHeaders` and `responseHeaders` to endpoint schema
 - Add flat, object-only rendering for header schema
 - Fix heading appearing for some empty content sections.
Updating styles for header sections

Styling for #4

- Adding style to distinguish header examples from body examples
- Replacing pt font sizes
- Moving layout styles out of theme stylesheet
Adding header example to readme

Issue #4
Updating fdoc fixtures to include header schema.

Issue #4
Updating block styles for headers / code sections.

Issue #4
Merge branch 'feature/document-headers' into develop