Make sure articles are available from `browseVignettes()`

[lazappi]

• The README links to pkgdown articles (e.g. anndatar.data-intuitive.com/articles/software_design.html) that are not available via browseVignettes().
• Please ensure all current documentation is discoverable from within R.
• If some pkgdown articles are out of date or deprecated, remove them from the README.

[lazappi]

The introduction and usage vignettes are available from R via browseVignettes(). The design and development vignettes are only available on the website (#342 moves them to articles/).

I think this is reasonable as they are mostly development focused and don't document using the package. The code in the development vignette also uses a bunch of packages that we don't want to add as dependencies.