Added xsltproc to the opentitan environment

[HU90m]

This enables one to build the docs with the opentitan environment.

Before the fix, I was getting this error:

2024-06-18 09:03:31 [INFO] (mdbook::book): Book building has started
Traceback (most recent call last):
  File "/home/hugom/r/lr/ot/main/util/mdbook_doxygen.py", line 97, in <module>
    main()
  File "/home/hugom/r/lr/ot/main/util/mdbook_doxygen.py", line 47, in main
    combined_xml = difgen.get_combined_xml(out_dir / 'api-xml')
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/hugom/r/lr/ot/main/util/mdbook/difgen.py", line 23, in get_combined_xml
    combined_xml_res = subprocess.run(
                       ^^^^^^^^^^^^^^^
  File "/nix/store/7hnr99nxrd2aw6lghybqdmkckq60j6l9-python3-3.11.9/lib/python3.11/subprocess.py", line 548, in run
    with Popen(*popenargs, **kwargs) as process:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/7hnr99nxrd2aw6lghybqdmkckq60j6l9-python3-3.11.9/lib/python3.11/subprocess.py", line 1026, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/nix/store/7hnr99nxrd2aw6lghybqdmkckq60j6l9-python3-3.11.9/lib/python3.11/subprocess.py", line 1955, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'xsltproc'
2024-06-18 09:03:32 [ERROR] (mdbook::utils): Error: The "doxygen" preprocessor exited unsuccessfully with exit status: 1 status