Add Security section to the site

[SensibleWood]

Referencing #59 and conversation therein...

Add a Section for Security support in OpenAPI, describing:

• The Security Scheme object and what is supported out-of-the-box.
• The security property and its application (globally, in a Path Item).
• Add nice graphvviz diagram showing where Security Scheme and security property live.

Writing style to approximate what's gone before based on mimicry rather than writing guidelines.

cc @lornajane

[SensibleWood]

This has now been merged so closing out the ticket