Patterns: migrate docs to new "guides" folder

[Eric-Arellano]

We're flattening the docs to be under a new "guides" section, like proposal 3 of https://github.com/Qiskit/documentation/pull/1294. This issue tracks renaming the files and setting up the new ToC.

This change needs to be well coordinated with internal changes like changing the top nav bar and adding redirects.

Git history should be preserved.