DEV DOCS: Add example of counting all words in all documents to our "contribute a feature" guide

[choldgraf]

The Contribute A Feature guide is a really useful starting point to understand where to access various information (like the current page AST) in the development process. However the guide focuses on a single page, whereas many contributions might involve the need to parse or traverse multiple pages.

I think an easy step forward there would be to add a short "addendum" or dropdown that showed the code that would be necessary to count the number of words across all pages in a MyST project instead of just on the current page. This could provide guidance to people that are interested in things like "replace an AST node on the current page with one that uses information from other pages".