Doc: LM: Reformat ly code.

Doc: LM: Reformat ly code.

- Use barchecks for multi-measure examples.
- Specify first duration of every measure/line.
- Quote unquoted \clef strings (eg., \clef "treble").
- Change "\TimeKey" to "\keyTime".
- Change "\new Staff << ... >>" to "\new Staff { ... }"
  for single-voice staves.
- Remove redundant durations and excess indents.
- Minor edits to some examples in content/formatting
  for clarity.