Updated docs + made mpl swatch look more similar to bokeh swatch

[story645]

Bit of all the things:

1. moved around doc & removed references to colorcet
2. added hook to customize mpl swatch to look closer to bokeh swatch
3. edited contribute doc
4. added PR template

[jsignell]

I'm rebasing this PR now and removing output from notebooks.

[story645]

Ack, sorry! #3 was such a mess that I forgot about this one. I can also rebase if you want.

[jsignell]

@story645 what do you think is the best way to ensure that people clear the output from the notebook before checking it in? A pre-commit hook?

[story645]

Probably, and also maybe adding it to the PR instructions and templates? I totally forgot that docs build systems run notebooks.

[jsignell]

I made the colormaps a gallery and condensed the two top level notebooks into one. Should make it easier to figure out.

[jsignell]

Deployed at https://pyviz-dev.github.io/contrib_colormaps/index.html
Will merge when green.

[ceball]

@jsignell I just happened to notice that there are In and Out prompts:

Is that via some setting (i.e. deliberate), or has something changed...?

[jsignell]

This site is using a different theme (alabaster) than the holoviz ones (sphinx_holovix_theme). I had assumed that the prompt numbers on our holoviz sites were being hidden by the theme, rather than by nbsite.

[ceball]

Ah right, that makes sense, thanks.