In-notes

• convertrules.py: Minor fixes and formatting

• NR: Minor typo

• titling.scm: Fix documentation strings and remove unused code

• Improve legibility of in-note regression test

• Rename in-note-padding paper variable to in-note-system-padding

  It has the same function for in-notes as footnote-footer-padding has for footnotes.

  Also add a convert-ly rule.

• Fix in-note handling

  • Add a new property in-note-padding that replaces footnote-padding.
  • Fix vertical spacing logic for in-notes. Previously, padding between in-notes wasn't handled correctly.
  • Add regtest for in-note configuration.

• NR: Document in-notes

  Also add an entry for changes.tely.

  Fixes issue #6399 (closed).