Added documentation for advance and distance. Various cleanup and fixes.

- Fixed multiple broken links to the documentation. Converted some of the
  links to internal references to the QuickBook documentation. Not all
  links converted because some of the QuickBook pages are not completely
  converted.
- Renamed utilities.qbk to type_traits.qbk and removed the part duplicated
  by concept_checking.qbk.
- Renamed traits.qbk to iterator_traits.qbk to better disambiguate with
  other type traits documented in type_traits.qbk.
- Converted some of the headings in iterator concepts to sections to be
  able to give those sections ids. This allows to reference those sections
  from other places in the documentation.
- In order to preserve the more compact pagination of the new sections
  (i.e. to have multiple sections on one page, like it was with headings)
  set chunk.section.depth=2 in the docs Jamfile.

doc/Jamfile.v2

@@ -16,7 +16,7 @@ boostbook standalone
     <xsl:param>boost.root=../../../..
     <xsl:param>toc.max.depth=3
     <xsl:param>toc.section.depth=3
-    <xsl:param>chunk.section.depth=4
+    <xsl:param>chunk.section.depth=2
     <format>pdf:<xsl:param>boost.url.prefix=http://www.boost.org/doc/libs/release/libs/iterator/doc
   ;