This improves several enhancements to the Aliki theme. And since Aliki
also became the default theme, we don't need to specify the generator name
anymore.

This reverts commit 26a9e0b4e31f7b5a9cbd755e0a15823a8fa51bae partially.

Use Aliki as the documentation website theme

This tells RDoc to not automatically link to the `ZJIT` module so we
don't need to keep escaping the word ZJIT in the documentation/comments.