feat: EPINIO-463 Contribution Docs

[johnlcos]

• Adds a new Contribution docs section for developers who want install Epinio or the Epinio UI on their machine locally in order to contribute to development
• Has sections for both the platform itself and the Vue UI
• Moves the docs that used to exist in those repos into the centralized docs site

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[dcharles525]

This looks good. One thing we probably should go ahead and do is update the backend development steps. The developer only really needs to do this step in order to deploy the backend changes if they already setup the UI. Perhaps we merge the two and have the user go through the pre-reqs first, then there is a backend and ui directory that have specifics. Also we do now have .devcontainer which is another way to run the backend.

The devcontainers docs are under another directory, with a few other files that we can talk through and move/remove depending on how stale they are.

While this expands the task a bit, it would be worthwhile just knocking it out now.