[wwwdocs] Use (more) CSS instead of direct formatting for our navigation bar

[Gerald Pfeifer]

In a lot of places I found guidance to use 
  margin-left:auto;margin-right:auto;
alas here really it is (only)
  text-align:center;
that works, which class="center" provides.

(For reference, https://css-tricks.com/centering-css-complete-guide/
is a good source of information on this.)

This does not get us all the way towards HTML 5 for the main page,
but a good step closer.

Committed.

Gerald

Index: style.mhtml
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/style.mhtml,v
retrieving revision 1.151
diff -u -r1.151 style.mhtml
--- style.mhtml 3 Sep 2018 19:34:55 -0000       1.151
+++ style.mhtml 8 Sep 2018 19:05:29 -0000
@@ -95,19 +95,19 @@
   <a href="<get-var BACKPATH>lists.html">Mailing lists</a><br />
   <a 
href="https://gcc.gnu.org/onlinedocs/gcc/Contributors.html";>Contributors</a><br 
/>
   <a href="<get-var BACKPATH>steering.html">Steering Committee</a><br />
-  <div align="center">
+  <div class="center">
   <a href="https://twitter.com/gnutools";>
     <img src="<get-var BACKPATH>twitter-bird-light-bgs.png"
       height="42" width="42" align="middle" alt="@gnutools on Twitter"
       />@gnutools</a>
   </div>
-  <div align="center">
+  <div class="center">
   <a href="https://plus.google.com/108467477471815191158";>
     <img src="<get-var BACKPATH>gplus-32.png"
       height="24" width="24" align="middle" alt="gnutools on Google+"
       />  gnutools</a>
   </div>
-  <div align="center">
+  <div class="center">
   <a href="https://my.fsf.org/civicrm/contribute/transact?reset=1&amp;id=57";>
     <img src="<get-var BACKPATH>donate-110-23.png"
       height="23" width="100" align="middle" alt="Donate to GNU Toolchain Fund"