docs: animated time encoding documentation and examples

[jonathanzong]

This PR contains documentation for animation features added in #8921.

• added doc comments to time encoding channel, animationDuration and framesPerSecond scale config (updates schema)
• add time channel to encoding.md documentation.
  • lists current restrictions with use of time channel
• add animation-related config to scale.md under new Animation subhead in Configuration section
• add two examples to the example gallery under new Animated header (after Interactive)
  • Gapminder frame animation
  • Hypothetical outcome plot
• removed the legend from the gapminder example so it looks cleaner

A problem before I ask for a review: the automatic vg2png thumbnail generation doesn't currently work with animated specs (it'll generate an image with without filtering, i.e. all the frames show at once). I imagine this is probably because the animated specs are getting run through an older version of the compiler before we added animation. I'd appreciate some guidance on what I need to do to fix this. We can't push the docs until then because presumably running the script will over-write the pngs with incorrect ones even if we put correct ones in manually.

[domoritz]

Please reference an issue so we know when this work is done and keep track of what needs to be done here.

[domoritz]

If this is not supported yet, we should not expose this in the spec. You can use @hidden if you still need it in the types.

[domoritz]

Suggested change

	- must explicitliy define a filter using that parameter
	- must explicitly define a filter using that parameter

[domoritz]

A problem before I ask for a review: the automatic vg2png thumbnail generation doesn't currently work with animated specs (it'll generate an image with without filtering, i.e. all the frames show at once). I imagine this is probably because the animated specs are getting run through an older version of the compiler before we added animation. I'd appreciate some guidance on what I need to do to fix this. We can't push the docs until then because presumably running the script will over-write the pngs with incorrect ones even if we put correct ones in manually.

Yeah, don't put manual ones in. I suspect something doesn't get cleared.

[domoritz]

I'm seeing warnings for Channel time should not be used with an unsorted discrete field. for the gap minder example. We should try to avid having examples throw warnings.