BUG: Make the documentation config file latex_preamble be a literal

Make the documentation config file `latex_preamble` be a literal string:
add the `r` prefix.

Fixes:
```
  File "/home/runner/work/tract_querier/tract_querier/doc/conf.py", line 229
    """
       ^
SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 1-2: truncated \uXXXX escape
```

raised for example at:
https://github.com/demianw/tract_querier/actions/runs/12170692299/job/33946161700#step:4:15

doc/conf.py

@@ -223,7 +223,7 @@
 #latex_use_parts = False
 
 # Additional stuff for the LaTeX preamble.
-latex_preamble = """
+latex_preamble = r"""
 \usepackage{amsmath}\usepackage{amsfonts}\usepackage{bm}\usepackage{morefloats}
 \usepackage{enumitem} \setlistdepth{10} \renewcommand{\Re}{\ensuremath{\mathbb R}}
 """