Migrate to GitHub Actions CI

[cbkerr]

Migrates signac-examples to use GitHub Actions. Also upgrades the HOOMD notebook to use HOOMD version 4.

Closes #66.

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[bdice]

@cbkerr I think this is ready. Please take a look when you have a moment, but I am planning to go ahead and merge once things pass so that I can update the examples in the documentation (which has been out of date).

[bdice]

Follow-up work is already captured in issues: we need to re-enable the flow.hoomd.lj notebook, which still uses HOOMD 2 (see #18). Also we should complete #64 by removing or updating the other outdated tutorials.

[bdice]

Docs are updated here: glotzerlab/signac-docs@956b2ff

edit: ...and they're live. Great!

[cbkerr]

@bdice I had a few questions

1. Why did you choose to run all projects within the CI job as opposed to making a separate action for each test project? I guess 3 min isn't too long to wait if repeatedly testing only one of the projects.
2. What happened to fix the import error we had on early tests on the line from mistune import BlockLexer?

[bdice]

1. If we expand the CI matrix to include more OS or Python versions, we'd end up with way too many jobs if we have (OS x Python x Projects). I decided the runtime was fast enough, and the previous CI was designed to run in a single job so I decided to just keep it in the same design. I also wanted to take advantage of run-tests.sh. That script already knows which projects to run or skip via the presence of .skipci files, and it also knows how to detect and install any additional requirements for each project.
2. The error ModuleNotFoundError: No module named 'mistune' was resolved by activating the conda environment. We overlooked an "IMPORTANT" note earlier from the setup-miniconda README. We hadn't noticed that the shell needed to be set to shell: bash -el {0}. b54fa1b