a way to consolidate documentation changes

In trying to figure out the current status of the `strip_prefix = "."` bugs (#354, #82), I noticed a documentation PR that adjusted the docs for `strip_prefix` in v0.8.0 but didn't do so in for v0.9.1 ([PR in question](https://github.com/bazelbuild/rules_pkg/pull/699)). See [the documentation site](https://bazelbuild.github.io/rules_pkg) and specifically the [v0.9.1](https://bazelbuild.github.io/rules_pkg/0.9.1/reference.html#pkg_tar) and [v0.8.0](https://bazelbuild.github.io/rules_pkg/0.8.0/reference.html#pkg_tar) versions of the pkg_tar docs.

It'd be nice to have an automatic way for docs to get carried into new versions automatically. Or to generate docs from the skylark code itself.

(I'm not sure what the latest state of skylark-to-markdown generation is or anything. I'm just a normal user who was confused if 0.9.1 had changed the behavior of `strip_prefix` or something)

(Also, I got those documentation pages from the "Documentation" link the README.)