Add a table of contents & quick description of each API in the API page #1478

A table of contents with anchor links should be added to the API module list to find content faster.

Also, we should add a short english description of what each API plugin can spefically do. This will make the Analytics API page a bit more digestible.

(In [4448]) Fixes #1478

• Adding one sentence to one paragraph description of each API (feedback on the actual test is most welcome!)
• This will be displayed on the API Reference page: http://piwik.org/docs/analytics-api/reference/
  that's why it's pretty important :)
• Adding table of contents in the API Page

(In [4454]) Refs #1478 linking to existing doc pages from the API descriptions