index: use a longtable

With version v2.2.0-pre1, the Revision History table starts to seriously
overflow in the pdf produced by LaTeX. Drop the sphinx tabularcolumns
directive and force a LaTeX longtable instead to fix this.
We adjust the columns widths to be reasonably close to the previous layout.

Fixes: #126
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>

source/index.rst

@@ -21,9 +21,10 @@ Creative Commons, PO Box 1866, Mountain View, CA 94042, USA.
    :alt: Creative Commons License
    :align: right
 
-.. tabularcolumns:: l c p{11.5cm}
 .. list-table:: Revision History
+   :widths: 15 10 75
    :header-rows: 1
+   :class: longtable
 
    * - Date
      - Issue