add info on checkpoint sync
* add detail to sync modes page * add code snippets * update code snippets
* add sync-modes page * update sync-modes page * replace frontmatter * update les link and line lengths * Apply suggestions from code review * update post-merge syncing description * clarify snap sync sequence * fix typo * Apply suggestions from code review Co-authored-by: Martin Holst Swende <martin@swende.se> Co-authored-by: Martin Holst Swende <martin@swende.se>