Web: import new web site from web-gop branch.

Web: import new web site from web-gop branch.

Overall structure by Graham, with many comments and suggestions from
-user.  Patrick McCarty worked extensively on the css and its
integration with texinfo.  Jonathan worked on the Introduction.
Patrick Schmidt did further work on the CSS and the News page.

I reworked the structure of files and directories to integrate it as
well as possible in Lily sources structure and build system.

Status/to-do of this draft:

* no PDF output -- best would be to generate per-page PDF and add
  "Print this page" links from HTML pages, which will be doable with a
  Texinfo parser;

* no big HTML page output: this makes little sense for a website, and
  the init file is currently not compatible with it: the images paths
  in the CSS would need to be adjusted to have both one big page and
  multiple pages outputs;

* HTML output: file names should be tweaked, which requires converting
  Jan's patch to texi2html script into a function in
  web-texi2html.init;

* SVG annotated images should be converted to PNG;

* examples of LilyPond scores are missing, they will likely go into a
  new directory e.g. Documentation/examples;

* top news in the HTML website and a RSS feed should be generated from
  Documentation/general/news.itexi;

* cross-references mechanisms between the new web site and the manuals
  should be set up;

* download links should be generated using update-version database;

* online and offline targets should be adjusted;

* support for translating this document is yet to be added;

* the CG should be updated.