doc: document self-referencing a package name

Added a section for "Self-referencing a package using its name" that
documents importing a package's own exports (this was missed when
adding the feature).

PR-URL: https://github.com/nodejs/node/pull/31680


Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Geoffrey Booth <webmaster@geoffreybooth.com>