Support for template substitution for `language` in `notfound_urls_prefix`

[m-aciek]

Hi, in Python docs hosted at https://docs.python.org/, we have multiple builds, for multiple languages. We'd like to be able to specify following configuration:

notfound_urls_prefix = '/{language}/3/'

that would resolve to a language (in pt-br format, lowercased, with a dash) used for the build (provided as -D language=XX).

Thank you!