Maintain the complete list of roxygen tags as Rd topics

[lionel-]

So they are included in the reference index:

I often have trouble finding my way in the roxygen documentation. Having a list of clickable links for each tag would be very helpful I think.

Here is how the index above is implemented, with contents shared in both Rd topics and vignettes: lionel-@eb6f82a3

It requires a solution to that issue so that aliases starting with @ are included in the reference index: r-lib/pkgdown#1436

[maelle]

related: #1080