doc: Improve layout of MISRA-C rules table

The layout of this table unreasonably assigns narrow widths for
for column(s) with lots of content, and wise-versa.

Define relative column widths to be more in sync with column
content. This will potentially produce more compact table, making
it more readable, clearer and more visually pleasing.

Signed-off-by: Aleksandar Markovic <aleksandar.markovic.sa@gmail.com>

doc/contribute/coding_guidelines/index.rst

@@ -52,6 +52,7 @@ severity and the equivalent rules from other standards for reference.
 
 .. list-table:: Main rules
     :header-rows: 1
+    :widths: 17 14 43 12 14
 
     * -  MISRA C 2012
       -  Severity