Fix duplicated external links in collections (#1001)

Fix external links and collections

The navigation should only display the external links once, after the links to pages that are not in collections.

The test for PR #876 at https://just-the-docs.github.io/just-the-docs-tests/navigation/external-links fails with v0.4.0.rc3,
and succeeds when the updated `nav.html` is added locally.

The docs need updating to clarify how the interaction between the collections feature and the external links feature is resolved.