[css-logical-props] Make margin properties use self's writing mode

The spec said to use the parent element's writing-mode/direction/
text-orientation to map margin-block-start to a physical margin
property. WebKit/Chromium/Gecko/Edge however use the own element's
writing-mode/direction/text-orientation rather than the parent's.
Since browsers use logical properties in their UA stylesheet and
the <dd> element uses margin-inline-start, so Web content that
uses <dd dir=rtl> where the parent is ltr would change behavior
if browsers were to change to use the parent element.

Ref.
https://logs.csswg.org/irc.w3.org/css/2016-05-11/#e685444
https://lists.w3.org/Archives/Public/www-style/2016May/0098.html

(Also remove Overview.html and let it be generated on the server.)

Author: zcorpan

css-logical-props/Overview.bs

@@ -253,7 +253,7 @@ the margin- and offset- block-start/block-end/inline-start/inline-end properties
 
   These properties correspond to the 'margin-top', 'margin-bottom',
   'margin-left', and 'margin-right' properties.
-  The mapping depends on the <em>parent element's</em> 'writing-mode',
+  The mapping depends on the element's 'writing-mode',
   'direction', and 'text-orientation'.
 
   <pre class="propdef">