Issue 3414: Mark c0-position as an internal grob property

Since its value is explicitly initialized from either
middleCClefPosition or middleCPosition for the relevant grobs
KeySignature and KeyCancellation, overriding or tweaking it has no
visible effect.
outside-staff-padding: just one pad; issue 2910

The 'outside-staff-padding of the object being placed on the outside is
left between it and any objects closer to the staff.
Issue 3424: Forgotten @code{} in pitch-scheme.cc

Wraps middleCOffset from doc-string of "ly:set-middle-C!"
(pitch-scheme.cc) into @code{}
Issue 3420: Stop Documentation/ly-examples/orchestra.ly from redefining markup commands
Updates to NR chapter 1
news: lilypondblog moved to new domain
Announces the LilyPond Blog in pondings.
Issue 3422: Changing EventChord to NoteEvent in scheme-tutorial.itely

Signed-off-by: David Kastrup <dak@gnu.org>
Updates to NR chapter 2