Better line numbers for markdown source generator diagnostics

[michael-hawker]

There are a few TODO items in the ToolkitSampleMetadataGenerator.*.cs files related to line numbers for diagnostics for some of the frontmatter/sample references.

This issue is to track fixing those to provide better line numbers pointing to where the error is coming from.

We'll have to re-perform a search/match I think to get the line location in the original file (if we have the text there).

Originally posted by @Arlodotexe in #85 (comment)

There's also an open question see here about whether or not we should emit a warning if there's no documentation provided (in an experiment)?