[fix] make docs: NameError: name 'DOC' is not defined

To reproduce the issue set base_url::

    diff --git a/searx/settings.yml b/searx/settings.yml
    index 841457b5e..4e282cb61 100644
    --- a/searx/settings.yml
    +++ b/searx/settings.yml
    @@ -72,7 +72,7 @@ server:
       bind_address: "127.0.0.1"
       # public URL of the instance, to ensure correct inbound links. Is overwritten
       # by ${SEARXNG_URL}.
    -  base_url: false  # "http://example.com/location"
    +  base_url: "http://example.com/location"
       limiter: false  # rate limit the number of request on the instance, block some bots

and build the docs::

    $ make docs
    SPHINX    HTML ./docs --> file:///800GBPCIex4/share/SearXNG/dist/docs


    DOCS      build build/docs/includes
    Traceback (most recent call last):
      File "searxng_extra/docs_prebuild", line 85, in <module>
        sys.exit(main())
      File "searxng_extra/docs_prebuild", line 31, in main
        f.write(page.content)
      File "/usr/lib/python3.8/contextlib.py", line 120, in __exit__
        next(self.gen)
      File "searxng_extra/docs_prebuild", line 81, in _instance_infosetset_ctx
        return DOC
    NameError: name 'DOC' is not defined

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>