Miller docs

Why Mkdocs

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.
Mkdocs vs Sphinx: these are similar tools, but I find that I more easily get better desktop+mobile formatting using Mkdocs.

pip install mkdocs (or pip3 install mkdocs) as well as pip install mkdocs-material.
The docs include lots of live code examples which are invoked using mlr, which must be somewhere in your $PATH.
Clone https://github.com/johnkerl/miller and cd into docs/ within your clone.

docs/src has *.md.in files containing markdown as well as directives for auto-generating code samples. Edit these files, not *.md directly.
Within a *.md.in file, lines like GENMD_RUN_COMMAND mark a live code sample: the command gets run, and its output included, when the file is processed.
The genmds script reads docs/src/*.md.in and writes docs/src/*.md.
mkdocs build reads docs/src/*.md and writes HTML files in docs/site.
Running make within the docs directory handles both of those steps.
TL;DR: just make docs from the Miller base directory.

In one terminal, cd to the docs directory and leave mkdocs serve running.
In another terminal, cd to the docs/src subdirectory and edit *.md.in.
Run genmds to re-create all the *.md files, or genmds foo.md.in to just re-create the foo.md.in file you just edited, or (simplest) just make within the docs/src subdirectory.
In your browser, visit http://127.0.0.1:8000
This doesn't write HTML in docs/site; HTML is served up directly in the browser -- this is nice for previewing interactive edits.

Create docs/src/foo.md.in.
Add a nav entry in docs/mkdocs.yml pointing at foo.md -- the filename must match the basename of your new .md.in exactly, or the built page won't be linked into the site.
First-time gotcha: docs/src/Makefile's genmds target only rebuilds files already listed in $(wildcard *.md). Since foo.md doesn't exist yet, a plain make within docs/src won't generate it. Two ways around this:
- Simplest: just use make docs from the Miller base directory (or make -C docs/src forcebuild) -- these force-run genmds on every *.md.in unconditionally, new or not.
- Or, from docs/src, run ./genmds foo.md.in directly, once, to create the initial foo.md. After that it's picked up by ordinary make runs like any other page.
Don't work around this by pre-creating an empty foo.md yourself (e.g. via touch): if its mtime ends up newer than foo.md.in, make's implicit %.md: %.md.in rule will see the target as up to date and skip regenerating it, silently leaving it empty.
git add both foo.md.in and the generated foo.md.

cd to the src subdirectory of docs and edit *.md.in.
make -C ..
This does write HTML in docs/site.
In your browser, visit file:///your/path/to/miller/docs/site/index.html
Link-checking:
- sudo pip3 install git+https://github.com/linkchecker/linkchecker.git
- cd site and linkchecker .

Do the publishing-build steps -- in particular, docs/src/*.md.in and docs/src/*.md are both checked in to source control.
- TL;DR: edit docs/src/foo.md.in and run make docs.
- If you don't want to do pip install mkdocs then feel free to put up a PR which edits a foo.md.in as well as its foo.md.
git add your modified files (*.md.in as well as *.md), git commit, git push, and submit a PR at https://github.com/johnkerl/miller.

Miller documents use the Oxford comma: not red, yellow and green, but rather red, yellow, and green.
CSS: the Mkdocs "material" theme is used, customized via docs/src/extra.css for Miller coloring/branding.