Update plug-in API docs to follow MDN format

[paulrobertlloyd]

Updates the plug-in API docs to follow a structure similar to that used on MDN. Roughly speaking, that means showing the syntax, briefly outlining the different properties and methods, then expanding below with examples, finishing off with links to related content.

In part, this should help address #652. Further changes will be needed in other parts of the documentation, but this is a good start.

Plan is to:

• update API docs for Indiekit.addEndpoint
• update API docs for Indiekit.addPreset
• update API docs for Indiekit.addStore
• update API docs for Indiekit.addSyndicator
• update API docs for IndiekitError
• update references to use new post type concept