Add Notation appendix for context mod identifiers.

Add Notation appendix for context mod identifiers.

* Documentation/notation/notation-appendices.itely:

  add node for context modifications

* lily/context-mod.cc:

  add type predicate ly:context-mod?

* ly/context-mods-init.ly (RemoveEmptyStaves):

  add docstring via \description

* scm/document-context-mods.scm:

  new file for generating context-mod documentation

* scm/document-identifiers.scm (identifier<?):

  move to documentation-lib.scm

* scm/documentation-generate.scm:

  load document-context-mods.scm and output documentation to
  context-mod-identifiers.tely

* scm/documentation-lib.scm (identifier<?):

  moved from document-identifiers.scm so it can be used in
  document-context-mods.scm too

* scripts/auxiliar/ref_check.tely:

  exclude context-mod-identifiers.tely

* scripts/build/website-known-missing-files.txt:

  add context-mod-identifiers.tely