Documentation tweaks

Changes here:

  - Mousing over an option shows a link icon, for linking directly to that documentation.
  - Compensate for fixed navbar in a better way.
  - Point tutorial at `dygraph-combined-dev.js`, which is more beginner-friendly.
  - Add a warning about supporting OldIE

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"Major websites, such as the New York Times". A link to NYT will confuse users.