user stories for roadmap development

[pmeier]

nebi has grown organically since its inception in a reactionist environment without a clear roadmap. This PR adds user stories that will drive the roadmap development.

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[smeragoel]

Went through the user stories doc, it's in a really good shape. I really like the glossary especially! A few suggestions:

1. The user stories slice personas by deployment context (solo / team / automation / cluster), but I don't know where to fit the notebook user (the data scientist on Nebari who just picks a Nebi environment as a Jupyter kernel and runs a notebook). We have a lot of kernel bugs (Workspace path not stored in database, kernels not discoverable #294, the launcher work), should we either add a stakeholder for them or add kernel stories into an existing persona?

2. There are no stories for editing an environment. Is that intentionally omitted because it's a basic story? A big chunk of what we've been designing (the save-and-install feedback work in [ENH] Provide feedback after "Save & Install" #327, the TOML/UI editing split in [FEAT] - Separate TOML and UI editing instead of a bidirectional toggle #413) is based on this story.

3. There are currently no failure / error-handling stories. Something like "As a developer, when an environment fails to build, I want to understand why and be assured my previous working version is intact