Pondings: Update URL to Prof Acim's LP book

URL had moved.
This is a more permanent link.
More consistent performance of breath marks. (issue 3821)

After tied notes, breaths take time only from the last note of the tie, so
e.g. { c4 ~ c8 \breathe } performs as { c4 ~ c16 r }, not { c4 r8 }.
This is consistent with articulations, and with how humans interpret
breaths after ties. This makes it possible to align simultaneous
breaths in multiple parts with different note lengths.

Ignore grace time when calculating the length of the note before a breath —
subsequent grace notes take time from the breath, not the previous note.
Let Chord_name_engraver record lastChord in internal context property

Using a context property instead of an engraver-internal variable
allows for more flexible behavior of identical chord suppression.
Issue 3835: Chord change detection in repeat alternatives

Let chord change detection in repeat alternatives happen in relation
to the chord active at the beginning of the first alternative.
Changes.tely - Document 4 new clefs

Document the 4 new Clefs from commit

476c0adb

Double G, Tenor G, Varpercussion and VarC
Doc: NR: Added @knownissue for midiinstrument

Issue 3823

@knownissue added in 3.5.1 (Creating Midi Files)

Midiinstruments (and other MIDI options) appear twice in the output
Web:Reworded UNIX Download page more strongly not to use distro-based packages

Issue 3829

Reworded 'Generic Packages or ....' section more 'strongly' to  get
users to _not_ use  distribution-specific packages but to use
the generic packages from lilypond.org.

Removed the 'Distribution-specific' Packages section completely.

Added some '32bit/64bit' labels for the generic package download links

Minor grammar and syntax edits
NR: Add verbatim to @lilypond example

NR 1.3.3 - Clef

Added a 'verbatim' to an @knownissue to better
illustrate the example