ReST.rst: added link targets, and refs for source code.
Sphinx.rst: more on source code highlighting.
References.rst: new link for C. Reller Memo
Sphinx.rst: new content, new crossrefs

-   New content:
    -    uses of roles
    -    more on only conditions
    -    introduce ifconfig
-   many new index entries
-   new refs targets
References.rst: checked and updated references
ReST.rst: improvements

-   updated sectionning
-   added difference Sphinx/Rest on code blocks
-   rewritten the include directive
-   many small improvements
Project.rst: added todo extension.
Project.rst: split between Project.rst and Extensions.rst,updated index.rst
Extensions.rst: more on math extensions.