Finish conversion of documentation to Sphinx

[gassmoeller]

We have almost finished the conversion of the manual to the Sphinx online documentation at aspect-documentation.readthedocs.org (see #4595), but a few points remain to be done. This issue is here to track the completion of these issues (and can be updated with additional points if necessary):

• The pdf version of the manual does not build cleanly (reported by @tjhei, can you open a PR with the fixes you already have? I can probably look into it, but would like to see what you already have)
• Units of input parameters do not appear correctly, because the si package is not available in Sphinx (see e.g. here
• It would be nice to recreate the functionality from the pdf manual to link from parameter file snippets in the documentation to the relevant input parameter documentation. This likely requires annotating the .prm files during the build process with the correct link to the parameter documentation.
• Generate an index for where keywords or input parameters are used in the documentation.
• ... let me know what else

[tjhei]

generating an Index?