Improvements for generated appendix section titles

• lily-library.scm: Remove map-alist-vals and map-alist-keys

  These non-public functions are unused since commit fd58a98a (in 2003).

• doc: Improve conversion from #:category to section name

  Use an alist to map categories to section/node names to produce better entries than, say, 'Other'.

• NR: Add link to section with dimension-related markup commands

  Fixes #6471 (closed).