split out large docs pages: Plotting, Reading and Writing Files

[dcherian]

Is your feature request related to a problem?

The "Plotting" and "Reading and Writing Files" pages are quite long.

Now that we are on pydata-sphinx-theme it would be good to split them out, and make the material more discoverable.

Describe the solution you'd like

I propose:

1. we split io.rst into three different files: netCDF/HDF; Zarr; and Others.
2. we splitting plotting.rst to: "Lines", "2D Plots", "Scatter, Quiver, & Streamplot", Faceting