docs: Use a separate top-level document for lAtEx

This screams about duplicate labels even *if this part isn't built!*

So we use another document that we ignore in other builders.

Blergh

doc_src/index.rst

@@ -183,10 +183,3 @@ If you have an improvement for fish, you can submit it via the GitHub page.
              design
              relnotes
              license
-
-.. only:: builder_latex
-
-   .. include:: tutorial.rst
-   .. include:: interactive.rst
-   .. include:: language.rst
-   .. include:: fish_for_bash_users.rst