Add Great Docs site (including workflow file)

[rich-iannone]

This PR provides the initial configuration of the Great Docs site (as great-docs.yml) and sets up a minimal docs directory. The User Guide materials were placed in user_guide and the benchmarks.qmd file was placed in the benchmarks dir for the time being (will re-integrate into the docs once the docs are building correctly).

The workflow file docs.yml should build the documentation as it currently successfully does on https://github.com/rich-iannone/great-docs.