Address #236, recreate XML files when xml-docs-test target is run directly
- must still `build initialize` between `git clean` and `build xml-docs-test`

nit: add description of xml-docs-test target
Add found TagHelperDescriptors on a Razor page to GeneratorResults.

- Ultimately this enables tooling to inspect what TagHelperDescriptors were found on a document and construct HTML schema based off of them.
- Added XML doc on the classes I touched that didn't have docs.
- Added [NotNull] to the result construct parameters.
- Added tests to validate that TagHelperDescriptors flow when found after parsing a Razor document.

#215