Add custom 404 page

[huonw]

This uses https://sphinx-notfound-page.readthedocs.io to add a custom 404 error
page, so that someone landing on our documentation can attempt to find something
useful, rather than having the completely link-less default RtD "maze" 404 page
(e.g. https://abasdfdas.readthedocs.io).

This gives two suggestions to a user who lands on an invalid page:

• using the sidebar
• doing a search, with a prefilled link by pulling out the alphanumeric parts of
  the URL path (ignoring the version section),
  e.g. /en/latest/demos/graph-classification/gcn.html gets turned into a link
  to a search for demos graph classification gcn (a potential improvement here
  would be to do that search directly in the 404 page)

Example:
https://stellargraph--1577.org.readthedocs.build/en/1577/demos/graph-classification/gcn.html

See: #1576

[codeclimate]

Code Climate has analyzed commit bd336e3 and detected 0 issues on this pull request.

View more on Code Climate.

[kjun9]

This is nice 👍 seems to work well