Reorganize docs and add a quickstart guide

[keithchev]

The documentation for the package currently consists of a series of jupyter notebooks. This PR:

• reorganizes these notebooks into a docs/ subdirectory.
• adds a quickstart guide to provide a quick overview of how to use the package.
• adds a documentation README with links to the notebooks, the quickstart guide, and the notebooks of example plots.

Note: github does not recognize that the style_usage.ipynb notebook was moved; it treats it as a new file, so there's no way to view the diff, which is annoying. (I was careful to move the file in one commit and modify it in a second commit, so the git history "knows" about the renaming; this must be a github thing).

Other changes

• move the notebooks in /plot_testing into /docs/examples.
• remove the PDF files of example plots that were previously generated by the style_usage notebook.
• minor edits in the style_usage notebook for clarity/succinctness.
• move ipykernel from a main dependency to a dev dependency (as it is only needed to run the documentation notebooks).

Style changes

These were made in discussion with Dennis.

• increase the default title size (so that it is not smaller than the default legend title font size).
• rename mpl.style_axis to style_plot for clarity.
• rename the color lightgray to gray to be consistent with the style guide. -rename brown_shades to warm_gray_shades and grey_shades to cool_gray_shades.
• renaming to use "gray" instead of "grey" everywhere.

PR checklist

• [x] Describe the changes you've made.
• [x] If you've added new software dependencies, make sure that those dependencies are included in the appropriate conda environments.
• [x] If you've added new functionality, make sure that the documentation is updated accordingly.

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[keithchev]

@mezarque I made one minor change after you reviewed to add numpy as an explicit dependency. Since numpy was already a transitive dependency (of matplotlib), nothing was broken, but because some apc modules import numpy, it's good to include it among the direct deps.