Hide the "dev docs" warning for internals/* pages

[tobiasmcnulty]

When you visit /en/dev/internals/* pages, you get a loud warning that "This document is for Django's development version, which can be significantly different from previous releases. For older releases, use the version selector floating in the bottom right corner of this page."

Since the internals pages are defined as the canonical version of the page for the contributing docs, we might want to remove this warning for this subsection of the docs.