Ship XML documentation files with the packages

[BrewingCoder]

None of the Tamp packages include an XML documentation file. Measured across a
populated package cache: 0 of 93 tamp package-versions contain any .xml doc
file; lib/net10.0/ holds only the .dll.

The practical effect is that there is no IntelliSense, no parameter
documentation, and no way to learn the API surface short of reflecting over the
assemblies or decompiling them.

Concretely, this cost real time and produced a wrong conclusion:

• I needed a target's plan sequence to keep running after one plan failed. I
  concluded the capability did not exist and reported to my team that it would
  need to be added to Tamp. It already existed as
  ITargetDefinition.FailureMode(FailureMode.Continue) in the version I was
  using.
• Separately, I wanted to run a CommandPlan imperatively and handle its exit
  code. I looked for Run/Execute/RunAsync on CommandPlan, found none, and
  abandoned the approach. The escape hatch is Tamp.ProcessRunner.Execute(plan, stdout, stderr, targetName) and ProcessRunner.Capture(plan, ...), both
  public and static. I only found them by enumerating Core types that accept a
  CommandPlan parameter.

Both are discoverable in one glance with doc comments, and neither was
discoverable without writing a reflection harness.

Suggested fix: set <GenerateDocumentationFile>true</GenerateDocumentationFile>
and include the generated .xml alongside each .dll in the nupkg.