[WIP] Initial implementation of inheritdoc support

[sharwell]

Quite a bit of work remaining here to handle edge cases:

• General inheritance implementation
  • [ ] Handle include elements: translate paths when a relative path will work, otherwise expand the element.
  • [ ] Handle recursive inheritdoc elements
  • [ ] Define behavior when a library contains <inheritdoc> elements in its documentation
• inheritdoc (DOC205)
  • [ ] Handle overridden root elements
  • [ ] Support nesting <inheritdoc> within a root element, e.g. within a <summary> element
  • [ ] Support the path/select attributes
  • [ ] Support the cref attribute
• autoinheritdoc (DOC206)
  • [ ] Exclusions for overridden root elements, which may be built on path/select support
  • [ ] Normalization of text
  • [ ] Define behavior for cases where included documentation is not available
  • [ ] Support the cref attribute
  • [ ] Support placement as a nested element

Closes #16

[codecov[bot]]

Codecov Report

Merging #64 into master will increase coverage by 0.01%. The diff coverage is 82.54%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
+ Coverage   82.26%   82.28%   +0.01%     
==========================================
  Files          71       78       +7     
  Lines        6012     6435     +423     
  Branches      418      449      +31     
==========================================
+ Hits         4946     5295     +349     
- Misses        912      977      +65     
- Partials      154      163       +9

[sharwell]

Now that <inheritdoc> is supported directly by Visual Studio and Roslyn, I'm not sure this feature is needed.