Docs before 2.6

[Graham Percival]

Here's the list of  things to be done before 2.6 comes out:

- (right before 2.6) change all "2.4.0" to "2.6.0" in 
Documentation/user/examples.itely
A simple regex will get them all and won't disrupt anything else, but 
we shouldn't do it
right now or else all the templates will be broken.

- preface.itely: Notes for 2.6.  This should be written by Jan and 
Han-Wen.

- invoking.itely: 5.3 Environment variables.  Do we need these any 
more?  I guess
we still have LILYPONDPREFIX and LANG, but I'm not certain if 
everything else
in this section is needed.

- INSTALL: this still lists TeX as a requirement for compiling.  Is 
that still true?


There's a few things that I can improve (how to do markup in lyrics, 
the new
and mysterious chapter 4, general cleanup, misc stuff from -user), but 
IMO
those aren't release-critical.

Thoughts?
- Graham