Create a Documentation Site for Ardalis.CleanArchitecture

Create an official documentation site for the template.

Currently the README(s) serve as all of the documentation for the template, but it would be much better organized in a proper docs site. I've built many similar docs sites using markdown and Hugo, and it would be a fairly straightforward task for a coding agent to produce an initial pass.

Once in place, things like migration guides (see #992 ) could live in the docs site as well.

Start by reviewing the repository's README files to inventory the existing template documentation, then read the issue's reference to migration guide #992. Use Markdown and Hugo to organize that material into an official documentation site; done means the current README content is available in the site and migration guides have a suitable home.

Written by the indexing model from the issue text.