Add path of the not found URL into the link to search related terms

[humitos]

If you go to this page

https://stellargraph.readthedocs.io/en/stable/demos%20page

you will see a 404 page with a link to quickly search "demos page",

https://stellargraph.readthedocs.io/en/stable/search.html?q=demos%2020page

It links to the regular search addin q=demos page.

Source code to generate the link: https://raw.githubusercontent.com/stellargraph/stellargraph/9370caed1137c2527523a39212072df1760ca00f/docs/404.rst