[API Doctor] Add a validation failure for occurrences of TODOs in topics

[Lauragra]

Some PRs that have been open for longer than 49 days contain files that haven't been fully authored and contain TODOs from the generated files.

PRs that contain files with TODOs shouldn't pass validation and the contributor should have to author the required content. Please add a validation check for occurrences of** **TODO placeholders in content.

The validation error can say something like:
Missing content on line n. Replace the TODO with the required content.