Add HTML5 color format regex to DOCtor-RST whitelist

This prevents the regex pattern in the validation documentation
from being flagged as a violation.

Author: OskarStark

.doctor-rst.yaml

@@ -123,3 +123,4 @@ whitelist:
         - 'End to End Tests (E2E)'
         - '.. versionadded:: 2.2.0' # Panther
         - '* Inline code blocks use double-ticks (````like this````).'
+        - 'the `HTML5 color format`_ (``/^#[0-9a-f]{6}$/i``). If it doesn''t match it,'