add simple content sanity checks

[Ryun1]

=== Checking content sanity ===
  [WARN] 'title' has trailing whitespace.
  [WARN] 'abstract' has a markdown heading of 182 chars (>80): "My amazing abstract limited to 2500 characters y amazing abstract limited to 2500 charactersy amazing abstract limited to 2500 charactersy amazing abstract limited to 2500 characters"
  [WARN] 'motivation' contains nested CIP-108 field heading marker for 'motivation' (e.g. '### motivation'). Field content shouldn't redeclare a CIP-108 field name as a heading.