[isabelle-dev] repository and components etc.

[Makarius]

This is another round of refinements concerning components and repository 
build process (Isabelle/0febe9e433dd):

   * A default configuration of Poly/ML is provided via
     Admin/etc/components/main, which assumes the usual init_components in
     ISABELLE_HOME_USER/etc/settings, as explained in README_REPOSITORY.

   * Other ML systems can be configured by overriding ML_SYSTEM etc. in the
     settings file after that init_components (which is best done first, so
     that manual settings can override the defaults).

   * Old Admin/init_components and Admin/components_old are discontinued.
     If you can't build anything from the repository anymore, see
     README_REPOSITORY.

   * Admin/Release/makedist has been replaced by the slightly more formal
     admin tool "isabelle makedist", which takes the local clone and its
     components as starting point, instead of going throught the central
     server at TUM (which was a historic artifact from the old CVS
     times).

   * The admin tool "isabelle build_doc" helps to populate the doc/
     directory, e.g. like this "isabelle build_doc -p IsarRef".

As the administrative infrastructure is becoming more and more 
comfortable, there is again the question which purpose is left for "the" 
development snapshot from http://isabelle.in.tum.de/devel/

It is still used for isatest runs, but it does not have to be made public 
as a crippled distribution that lacks most add-on components.


 	Makarius