This is to make clearer what operations do what, hence facilitate the selection of the right st_*() function.

This work will require a few iterations to continue, but thought I'd send this PR first to see if there is interest.

The end goal is to combine documentation that applies to many functions in a help topic while separating out the pieces of docs that only apply to a single function to avoid clutter in the doc.

The goal is to have multiple documentation topic and use roxygen2 tags to reuse documentation (like @inheritParams.

Also added a rough classification in the pkgdown index for #2176 (can still be improved)

Fix #2428, seems like this was just a documentation issue!
fix #2176