[Feature request] Provide doc comment coverage metrics for code coverage tools

It would be nice if DocFX could generate a "coverage" report, or some form of structured metadata output that could be used by other coverage tools, e.g. Resharper, OpenCover, NCover. This report or data would provide summary stats of the percentage of accessible code covered by documentation comments in the code, or by entries in separate documentation files, in a form similar to test coverage data.

Options could eventually include:

- whether to include `internal`, `protected`, or `private` entities, perhaps using only `public` by default;
- detecting and reporting where repeated doc strings could be replaced with `/// ` (although, perhaps that is a [StyleCop Analyzer](https://github.com/DotNetAnalyzers/StyleCopAnalyzers) feature); and
- quality indicators, e.g. whether the documentation comments are complete, incomplete, missing `` tags, present but empty, and so on.

Forgive me if this data is provided somewhere already, but I can't seem to find it anywhere.