Better line numbers for markdown source generator diagnostics

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 https://github.com/CommunityToolkit/Labs-Windows/pull/85#discussion_r864251696_

There's also an open question _[see here](https://github.com/CommunityToolkit/Labs-Windows/pull/85#discussion_r864253309)_ about whether or not we should emit a **warning** if there's no documentation provided (in an experiment)?