Docs about animations/updating plots

[asinghvi17]

I've been doing some animations with newcomers to Makie recently, and noticed some gaps in the documentation which we should probably fill.

This is mainly a set of notes for when I have the time to do this properly, but if others want to contribute by writing some of this up, that'd be great!

1. What are the contents of a Plot? How can
2. How do you update the value you passed to a plot directly, if it isn't already in the appropriate format?
   E.g. I have a type MyType for which convert_arguments is defined. However, setting plot[1] = MyType(new_args...) doesn't work.
   Solution: set plot.input_args[1]
3. We need to reorder the docs - tutorials should be on the top, followed by examples and documentation.
4. Animations have a tutorial but it's not in the tutorials section - that should also be moved there.
5. We should probably have a coherent, in-depth tutorial on the basic attributes - color, colormap, linewidth, marker, markersize, space, etc.

Will add more as I go along.