doc: outline for development section

Here is a proposal for pages in the development section:

1. [ ] Project structure
2. [ ] extendr macros
3. [ ] The build cycle
4. [ ] `Robj` - R's universal SEXP
5. [ ] Data types, scalars, vectors, and missing values
6. [ ] R Lists
7. [ ] Rust structs and hashmaps
8. [ ] R attributes, names, and S3 classes
9. [ ] R data frames and matrices
10. [ ] Condition handling
11. [ ] External pointers

I want to suggest that we move examples of working with specific Rust crates like tokio into blog posts. Then we should introduce two new sections at the same level as "development" and "publishing," specifically "features" and "advanced topics." Features would include all the gated crate integrations like serde and faer, and advanced topics would be for stuff like metaprogramming.

What do you all think? We already have a lot of this content. It just needs to be cleaned up, organized, and updated to be consistent with extendr 0.9.0 and rextendr 0.5.0.