Implement changes review Els

[wlangera]

Changes after review and meeting with @ElsLommelen (#7)

• update new function name autoqmd_insert_children() in vignette
• improve and/or add error and warning messages for users
• add argument child_labels to autoqmd_prepare()
• placeholder message above include part and only add once (maybe check if there is no comment above), remove feature from post render
• test dependency stamp in child files with freeze (maybe add freeze immediatly to child yaml, but what if there is already a yaml --> can also be tested for warning generation by finalise combination freeze and delete)