manual: use historical nixpkgs revision to obtain needed xls files

[sternenseemann]

https://github.com/NixOS/nixpkgs/pull/212289 removes the xls files we relied on in favor of a new solution that goes from options.json to docbook using a python script. This commit is a band-aid approach of pulling in the necessary files from an old nixpkgs revision to fix the build of the manual until we can solve it properly.

Since the intention of upstream is to get rid of docbook for good, it probably makes sense to port our module documentation to markdown already when porting our manual build process to the new toolchain.

cc @aszlig