chore(docs): Split up CSS Blocks Architecture documentation into high-level vs specific.

All the info here in the existing documentation is great - but hard to parse if you're new to it.

A high-level overview that links to more specific deep dives would help comprehension and readability. It would also provide an easy way to find what is missing and needs to be filled in.

Start by locating the existing CSS Blocks Architecture documentation and reviewing how its material is organized. Separate the current content into a high-level overview and linked, specific deep dives; the work is done when the overview is easier to scan and missing topics are apparent.

Written by the indexing model from the issue text.