Integrate the generation of Internal and Online documentation with roxygen2/pkgdown

[christophergandrud]

I think we should convert the Zelig online documentation generation to pkgdown. This has a number of advantages:

• Easily enable close integration with the internal documentation, which is currently different from the online documentation in ways that are potentially confusing to users

• Closer test integration as the online docs could be tested every time Zelig goes through a build check. This would enable more accurate documentation (e.g. docs that don't include errors) and provide another "test surface" for finding errors.

• Simplify package development toolchain.

[christophergandrud]

Ongoing in: https://github.com/IQSS/Zelig/tree/pkgdown