docs: conf.py: properly escape backslashes for latex_elements

There are some syntax warnings returned by Python:
"""
yocto-docs/documentation/conf.py:162: SyntaxWarning: invalid escape sequence '\P'
  'passoptionstopackages': '\PassOptionsToPackage{bookmarksdepth=5}{hyperref}',
yocto-docs/documentation/conf.py:163: SyntaxWarning: invalid escape sequence '\s'
  'preamble': '\setcounter{tocdepth}{2}',
"""

Backslashes must be doubled in Python string literals to avoid
interpretation as escape sequence, c.f.
https://www.sphinx-doc.org/en/master/latex.html#module-latex.

Cc: Quentin Schulz <foss+yocto@0leil.net>
(From yocto-docs rev: a78d9bde7f90f860aa969b7d8e47350de9106183)

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/conf.py

@@ -159,8 +159,8 @@ html_last_updated_fmt = '%b %d, %Y'
 html_secnumber_suffix = " "
 
 latex_elements = {
-    'passoptionstopackages': '\PassOptionsToPackage{bookmarksdepth=5}{hyperref}',
-    'preamble': '\setcounter{tocdepth}{2}',
+    'passoptionstopackages': '\\PassOptionsToPackage{bookmarksdepth=5}{hyperref}',
+    'preamble': '\\setcounter{tocdepth}{2}',
 }
 
 # Make the EPUB builder prefer PNG to SVG because of issues rendering Inkscape SVG