documentation: Use Sphinx for building HTML documentation

[asullivan-blze]

What and why?

Implements Sphinx to have some HTML-based documentation, so referencing documentation that is
not directly in Python source code files is possible. While this branch is not intended to
fully document all available functions, this does provide the necessary framework which
allows future documentation of the code -- within the code itself by way of docstrings -- to
auto-generate updated documentation.

Additionally, while not strictly needed -- and I'll probably look into making this a
change that's available in boardwalk/boardwalkd themselves by default -- I built some
tooling to make pretty HTML output of the --help documentation for commands using
rich-click. Right now this isn't baked into the existing documentation build process,
though I might look into seeing if it can be in the future. (I made an attempt, at
least, but the Bash script which does this was the 'good enough' compromise, since
it felt like I was fighting with Python for an excessive length of time to get it to
do what I wanted it to do.

Dependabot updates

As a bit of an enhancement I found while looking into how to use workflows to publish pages,.github/dependabot.yml was created to allow monitoring of not only the Python dependencies, but also the versions of the GitHub Actions.

How was this tested?

• Primary testing was done using Sphinx's Makefile at docs/Makefile, via make html.
• Another test phase was when the primary Makefile in the repository root had make docs added into it as a target (which just chains to the Sphinx Makefile).

Checklist

• Have you updated the version in the [tool.poetry] section of
  the pyproject.toml file (if applicable)?
  • Though this technically isn't a version-worthy bump, but I'll bump it regardless, since this is a decently-sized change.

Closes internal ticket SVRENG-619.

[asullivan-blze]

I've opted to narrow the scope to just adding the ability to build documentation via Sphinx; if a developer needs the documentation, it's easy enough to build, so let's not add complexity to the GitHub workflow process.

[agouldingbb]

LGTM