Documentation request re slots, compose, viewports, templates, parts etc. from Gitter

Posting on behalf of
> Alex Dresko @alexdresko

"Heads up for anyone wishing to making life easier for new devs -- Please document the relationship amongst: replaceable parts, templates, compose, viewports, named slots, and layouts. I've read about as much as I can find on each individual subject, but haven't found anything that really ties everything together. It's fairly confusing. At this point, I'm just going to dive in (commit first) and see what combination of features gets me what I need."

Good suggestion.

Start by reviewing the existing documentation for replaceable parts, templates, compose, viewports, named slots, and layouts. Map how these concepts relate in a single newcomer-focused explanation, with examples showing when each feature is used. Done means the relationships are documented together rather than only described separately.

Written by the indexing model from the issue text.