doc: Mark preamble variable as raw string
doc: Mark preamble variable as raw string

Importing `doc/conf.py` fails with Python 3.7:

```
>>> from conf import Mock
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "doc/conf.py", line 454
    ''',
SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 5-6: truncated \uXXXX escape
```

Therefore mark the string as raw due to the backslashes.

Signed-off-by: Benjamin Drung <86a6db991ec5c1a96f98c92b98df9d547d7c8c84@cloud.ionos.com>

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt