inputs edits

[mbostock]

This…

• Changes the inputs page to focus more on the concept of inputs, and less on Observable Inputs
• Adds an example of using Observable Plot as an input!
• Describes when to use Generators.input apart from view
• Moves the overview (usage) of Observable Inputs to the Observable Inputs page (/lib/inputs)

Still to-do:

• Trim the Observable Inputs page (and fix errors I introduced)
• Add something to the individual inputs pages that ties them more strongly to Observable Inputs

[Fil]

looking good so far!

[allisonhorst]

Nice!