Add options for the Julia binary and project path

[JamesWrigley]

This makes using JuliaPkg optional. Also sneaked in a change to use [sources] in the docs project.

Tested manually. Should fix JuliaPy/pyjuliapkg#56.

[cjdoris]

This looks good thank you! A few comments:

• Can we make the option be just exe not exepath? For consistency with PythonCall. Keeping it as exepath internally is fine.
• Why rename project to opt_project?
• I'd rather that using juliapkg was all-or-nothing. That is, either you have to set both of these options or neither. The reason being that the project and executable are coupled - the project was created with a specific version of julia.
• It's true that the project must contain PythonCall, but even stronger, it must be at exactly the same version as juliacall. Can you put this in the docs? I think this already gets checked by PythonCall so an error will happen if you get it wrong.

[JamesWrigley]

Roger that, I think all of those are fixed in 69b90ca (tested again manually).

(BTW I like using fixup commits during review :) if you think this is ready then I can rebase the branch, or you can squash merge)

[cjdoris]

Don't worry, I always squash.

Looking good.

Main other thing is testing. If you're familiar with GitHub CI can you add a case to the test workflow that uses these new env vars? The PythonCall tests (in the same file) do a similar thing to test against either CondaPkg or a pre-installed python. You'll have to conditionally do the install-julia action, followed by running a command to set up a Julia project, and then set these env vars when running the tests. If you're unsure I can do it when I get some time.

[JamesWrigley]

Got there in the end 😅 I tried to keep the actual test command contained in one step but it was tricky to set the environment variables correctly so I split it into one step for JuliaPkg and another without. Turns out that CI was already running the setup-julia action so I changed it to only run when we're not using JuliaPkg.

[cjdoris]

Looking good just a couple of small tweaks please.

[cjdoris]

I made a few little tweaks before merging. Great work, thank you!