Miller Sphinx docs

Why use Sphinx

Connects to https://miller.readthedocs.io so people can get their docmods onto the web instead of the self-hosted https://johnkerl.org/miller/doc. Thanks to @pabloab for the great advice!
More standard look and feel -- lots of people use readthedocs for other things so this should feel familiar
We get a Search feature for free

You need pip install sphinx (or pip3 install sphinx)
The docs include lots of live code examples which will be invoked using mlr which must be somewhere in your $PATH
Clone https://github.com/johnkerl/miller and cd into docs/ within your clone
Editing loop:
- Edit *.rst.in
- Run make html
- Either open _build/html/index.html (MacOS) or point your browser to file:///path/to/your/clone/of/miller/docs/_build/html/index.html
Submitting:
- git add your modified files, git commit, git push, and submit a PR at https://github.com/johnkerl/miller
A nice markup reference: https://www.sphinx-doc.org/en/1.8/usage/restructuredtext/basics.html

CSS:
- I used the Sphinx Classic theme which I like a lot except the colors -- it's a blue scheme and Miller has never been blue.
- Files are in docs/_static/*.css where I marked my mods with /* CHANGE ME */.
- If you modify the CSS you must run make clean html (not just make html) then reload in your browser.
Live code:
- I didn't find a way to include non-Python live-code examples within Sphinx so I adapted the pre-Sphinx Miller-doc strategy which is to have a generator script read a template file (here, foo.rst.in), run the marked lines, and generate the output file (foo.rst).
- Edit the *.rst.in files, not *.rst directly.
- Within the *.rst.in files are lines like POKI_RUN_COMMAND. These will be run, and their output included, by make html which calls the genrst script for you.
readthedocs:

Let's all discuss if/how we want the v2 docs to be structured better than the v1 docs.