Requirements, besides pandoc and sphinx:

    pip3 install beautifulsoup4 toolz lxml pandocfilters sphinx-variations

To convert, first grab an xml export of the manual:

    rm -f dump.xml
    wget https://www.panda3d.org/manual/dump.xml

Extract pages from it and convert them to RestructuredText:

    python3 foo.py dump.xml

Now for the sphinx step:

    make html

The result will be available at: build/html/main-page.html