docs: Avoid use of rules_pkg

I'd like to propose that rules_oci avoid explicit or implicit endorsement of rules from rules_pkg, particularly `pkg_tar`. Where documentation and examples currently refer to `pkg_tar`, use [the `tar` rule from bazel-lib](https://github.com/bazel-contrib/bazel-lib/blob/main/docs/tar.md) instead.

Rationale: in addition to the advantages of `tar` listed in the bazel-lib documentation, rules_pkg is poorly maintained and has surprising behaviors such as [flattening directory trees](https://github.com/bazelbuild/rules_pkg/issues/354) and omitting runfiles by default, which seem unlikely to ever be fixed.

Search the rules_oci documentation and examples for references to rules_pkg or pkg_tar, then review bazel-lib's tar rule documentation linked in the issue. Replace the relevant references with bazel-lib's tar rule and verify that no explicit or implicit pkg_tar references remain.

Written by the indexing model from the issue text.