Fixes and improvements to documentation content and build.

This merge request has two goals: to fix up the documentation build process so that the website and other documentation can be built on *nix, MacOS and MSYS2/MINGW; and to tidy up and improve the content so that it does not contain inaccurate information or broken links. The main changes are:

• The -Ddocs configuration option now allows the value auto to build or not based on installed pre-reqs. The default remains false
• A new config option -Dgtk-doc=<true|false> controls the build of the gtk-docs.
• The website documentation builds in its own subdiretories in the build tree docs/website. This should be complete including images and css files so that the static website can be more easily deployed. Only docs that can be accessed from index.html or its children are located in the website directory orphaned documents are built in the docs dir.
• Docs that are clearly obsolete have been moved to docs/obsolete in the source tree and are not built.
• Enscript has been replaced with source-highlight for the source code highlighting as the latter is called automatically from asciidoc.
• The build has been extensively updated.
• All of the pages in the static website now have the GEGL logo at the top which acts as a clickable link to the home page.
• All of the website documents have been edited for spelling / grammar / formatting issues and all dead links have been fixed or removed.
• The build and development pages have been extensively updated to reflect the current state of development.
• The gtk-docs are normally built during install - if they are built manually they will be located in the docs/reference directory of the build tree.

Should be applied before merge request !91 (merged)