Add unicode multiplication

Since checkmark already exists, it makes sense to have this too.
Both signs are used as "yes" or "no" replacements in text UIs.

sphinx/util/texescape.py

@@ -29,6 +29,8 @@ tex_replacements = [
     # map special Unicode characters to TeX commands
     ('✓', r'\(\checkmark\)'),
     ('✔', r'\(\pmb{\checkmark}\)'),
+    ('✕', r'\(\times\)'),
+    ('✖', r'\(\pmb{\times}\)'),
     # used to separate -- in options
     ('', r'{}'),
     # map some special Unicode characters to similar ASCII ones