Create more structured documentation for 'Build your own image'

[jnywong]

Context

We need documentation to accompany the 'build your own image' feature 2i2c-org/infrastructure#5064.

We are working with collaborators from VEDA to upstream docs to repo2docker: NASA-IMPACT/veda-docs#186

Follow-up task to 2i2c-org/infrastructure#5343.

Tasks

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1. Coordinate with Add docs for build your own image feature NASA-IMPACT/veda-docs#186
2. Upstream docs to repo2docker
3. Add a basic, user-facing overview that covers the UX from the JupyterHub perspective (as opposed to the BinderHub UX) with some background on caching, etc.
4. Add a small FAQ section that can be updated over time
5. Curate a resource list that references upstream docs, e.g. repo2docker
6. Document how to test JupyterHub images locally #229 (caveat: a comprehensive guide to image-building is not within the scope)