Document ReqnrollGenerationTaskFactory and AssemblyTaskFactory

[Copilot]

AssemblyTaskFactory is used by the code generation MSBuild task but was never documented, despite being key to resolving issues with nested and parallel MSBuild execution.

Documentation added (docs/installation/configuring-build.md)

• New section explaining the ReqnrollGenerationTaskFactory build variable and its default behavior:
  • TaskHostFactory on Windows — runs the task out-of-process to avoid Visual Studio locking the task assembly and to prevent conflicts when MSBuild is invoked in a nested or parallel fashion.
  • AssemblyTaskFactory on non-Windows platforms — loads the task assembly directly in-process, since TaskHostFactory isn't supported everywhere.
• Example showing how to override the default (e.g. forcing AssemblyTaskFactory on Windows):

<PropertyGroup>
  <ReqnrollGenerationTaskFactory>AssemblyTaskFactory</ReqnrollGenerationTaskFactory>
</PropertyGroup>