[docs] Clarify .NET 11 runtime-specific build properties

[vitek-karas]

Summary

• add a page-level boundary for Mono-specific build properties in .NET 10 and earlier versus CoreCLR in .NET 11
• document NETSDK1242 for UseMonoRuntime=true on .NET 11 iOS, tvOS, and Mac Catalyst while keeping the existing macOS CoreCLR-only workload error distinct
• scope the MAUI UseInterpreter=true Debug behavior to .NET 10 and earlier and clarify that MtouchInterpreter and UseInterpreter configure the Mono interpreter, not the CoreCLR interpreter
• preserve the existing warnings from [msbuild] Detect Mono-only build properties when not using the Mono runtime #26042 and leave MtouchLink and TrimMode unchanged

Validation

• git diff --check
• local Markdown link and anchor checks
• exact text/count assertions for the runtime boundary and both MAUI notes
• unchanged-section assertions for EnableSGenConc, MonoUseCompressedInterfaceBitmap, MtouchEnableSGenConc, MtouchUseLlvm, MtouchLink, and TrimMode
• single-file scope, prohibited-heading, collective-wording, and trimming-setting assertions
• cited the focused existing ProjectTest.MonoOnlyPropertyWarning and ProjectTest.MonoOnlyPropertyError coverage; these macOS workload tests were not run from the Windows worktree

Release evidence

Current main at 03e37082495026ce93f511a01fc07ef7e2170279 contains the #26042 warning behavior and #26468 (c7c6bba288295c08a6d125338f5bf5849d3f5bbc). The available release/11.0.1xx-rc.1 branch at 4d35fc6aedc02ef4bb9ecdae170cb8132be0a5d6 contains #26042 but does not contain #26468, and no final .NET 11 release branch is currently available. This PR follows the approved DOC-018 assumption that #26468 ships in final .NET 11; that ancestry must be rechecked before merge.

The .NET SDK gate currently raises NETSDK1242 for .NET 11 and later iOS, tvOS, and Mac Catalyst projects with UseMonoRuntime=true; macOS is excluded from that gate and retains the workload's Only CoreCLR is supported on macOS error. The .NET MAUI net11.0, release/11.0.1xx-rc1, and release/11.0.1xx-rc2 targets set UseInterpreter only when UseMonoRuntime=true.

Coordination

Synchronization to dotnet/docs-mobile is intentionally deferred to the coordinated DOC-018 public documentation PR.