feat: unified Python + R documentation sites (multi-language adapter)

We'd like to add support for unified documentation sites for projects using multiple programming languages (starting with the Python + R combination) so packages like shinychat and querychat can migrate their sites.

In both cases, the sites have custom landing pages that ask a user which sub-site (Python or R) to navigate to. We could make it so that there's a single Great Docs site that unifies both (one navbar, search index, and theme). Internally we have this spec'd out and this involves:

• a new sections: config key
• an R adapter that parses roxygen2 output to produce the same intermediate representation as the Python pipeline
• a section selector widget in the navbar for switching between languages

Both posit-dev/shinychat and posit-dev/querychat are monorepos with pkg-py/ and pkg-r/ subdirectories and are nice test cases for this work. We need to: (1) start with an internal IR refactor, (2) prototype a working Python + R site for shinychat or querychat from local paths, and (3) add cross-repo git source resolution (this is for getting one docs site built from sources in multiple repos).

cc: @gadenbuie

Start with the internal intermediate-representation refactor and review the sections: configuration requirement. Prototype a unified site from local pkg-py/ and pkg-r/ paths using posit-dev/shinychat or posit-dev/querychat, then investigate cross-repo git source resolution. Done means a working Python + R site with shared navigation, search, theme, and a language selector.