add not found extension to requirements.txt

Author: acharraggi

docs/requirements.txt

@@ -11,4 +11,5 @@ python-git-info==0.8.3
 sphinxcontrib-mermaid==0.9.2
 sphinx-hoverxref==1.3.0
 sphinxext-rediraffe==0.2.7
-git+https://github.com/FIRST-Tech-Challenge/ftcdocs-helper@main#subdirectory=linkcheckdiff
+sphinx-notfound-page==1.1.0
+git+https://github.com/FIRST-Tech-Challenge/ftcdocs-helper@main#subdirectory=linkcheckdiff