Adds `net6.0` as a target framework to mdoc. The artifacts from the
`net6.0` build will be output to `bin/$(Configuration)-net6.0`.
Mono.Cecil has been bumped from `0.10.0-beta5` to `0.10.0`. This update
appears to have introduced a change in behavior in F# doc generation,
and the expected output in `PatternMatchingExamples.xml` has been
updated to reflect that.
The mdoc NuGet package has been updated to include both `net471` and
`net6.0` tools. This can be considered a breaking change, as the path
of mdoc.exe has been changed to include a folder named after its target
framework.
