add example for installing FPF plugin components that aren't published to NPM

[brian-smith-tcril]

I had a conversation with Felipe where he asked:

Do you know of any tutor plugin that loads the code of a UIslot without requiring that the code is first published to NPM?

I didn't know any examples. The README examples for the slots all use inline RenderWidget code, and the other examples use NPM packages.

The solution used for openedx-ai-extensions ended up being Tutor's build-contexts.

https://github.com/openedx/openedx-ai-extensions/blob/main/tutor/openedx_ai_extensions/plugin.py#L32

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I definitely think "make a plugin that utilizes FPF slots but doesn't have frontend code published to NPM" is a reasonable use case that we should (and apparently already do) support, so it'd be great to document that.

[felipemontoya]

In short, what we did was to update the tutor installation to also include the sibling directories. I supose we could have done it in setup.py, but we took the opportunity to move into hatch_build.py.

When completed, doing pip install <route> would copy the current version of backend and frontend into the environment. We added a bit of logic to the tutor plugin to account for differences when using regular mode and editable mode (-e) so the correct path is added to the build-contexts.

Finally, we added an MFE hook in the pre-npm and post-npm stages of the build to install the code as a dependency.