docs: fix not found pages and version fallback
Created by: bobheadxi
All not-found pages and missing version pages has thrown a template error a la template error: template: root:3:28: executing "seo" at <.Content.Doc.Title>: nil pointer evaluating *docsite.ContentPage.Doc
since https://github.com/sourcegraph/sourcegraph/pull/25460 . This PR adds more safety checks around the template so that if we land on an empty page we don't attempt a nil dereference and error out. This should fix not-found pages (see below) and missing version fallbacks (which I will test when live)
Closes https://github.com/sourcegraph/docsite/issues/81 (this is not a docsite issue but a template issue)