add Documenter docs

[visr]

Wanted to try setting up a documentation page. I already added some deployment keys, so if this is merged it should build the first documentation. (it builds locally)

Other than moving the basic example to the docs, and adding docstrings, there are no functional changes, just docs.

[yeesian]

LGTM. Builds locally for me and tests are passing. Sorry I'm quite behind: why/when are the toml files required? And how are they generated?

[visr]

So this is just a project in the docs folder, not for the package. The reason it is advised to use them is https://discourse.julialang.org/t/psa-use-a-project-for-building-your-docs/14974.

The manifest file is technically not needed but records a known-to-work series of packages that the docs can be created under, so may come in handy in the future.

[yeesian]

Thanks for the explanation! I see you've already updated it for the Documenter 2.0 changes.

Just for clarity: that makes we will

• have stable doc builds, and
• need to explicitly update our doc build dependencies.