doc/stdlib: fix the html charset in header.html and co
doc: improved xhtml compatibility (cover, header,...)
doc/stdlib: fix the xhtml validity of the index-list template
coqdoc: fix a few issues with xhtml validity

 - For the style of identifiers, coqdoc was using a 'type' attribute of
   tag <span>. But this attribute isn't a legal attribute of tag <span>
   according to the xhtml norm. Instead, I propose to use 'title' for that.
   The coqdoc.css now supports both approaches.

 - The names of inner links (cross references #foo) were
   containing arbitrary characters (in the case of a notation string).
   For instance in Utf8_core : <a name=":type_scope:'∀'_x_'..'_x_','_x">
   Instead, when strange characters are detected, we now hash the
   string via Digest, and use this hexa hash as html label.

 - And some whitespace before />