Fix link to LeveneHaldane.pdf in FunctionRegistry

[jbloom22]

No description provided.

[danking]

@jigold is there a way to link to this PDF by name rather than by path?

[jigold]

Two options:

1. Add the file path to html_extra_path in conf.py and then the file will be in the docs directory upon building

2. Make a new directory in python/hail/docs/, move the PDF file and image files there, and then add the new directory path to html_extra_path in conf.py.

[danking]

#2 sounds right to me. In general, each version of the docs (right now, just devel and 0.1) needs it's own version of all assets (including the Levene-Haldane pdf).

[jbloom22]

Closing in favor of #2205