Self-host sample source code pages

[TimPurdum]

Summary

• Source Code pages in the sample hosts now read .razor / .razor.cs from embedded resources in Core.Sample.Shared and Pro.Sample.Shared instead of fetching .razor.txt from docs.geoblazor.com at runtime.
• Core hosts (Wasm, WebApp Server+Client, Maui) gain the Source Code button — previously Pro-only.
• The DocsUrl plumbing (build script flag, JSON emission, dev appsettings override) is retired.

Why

The runtime fetch coupled the samples to an external host that has to be online, CORS-friendly, and kept in lockstep with whatever ships in this repo. It also broke offline scenarios — most notably the MAUI hosts. Embedding the source in the assembly that compiles it eliminates the network hop and the drift risk.

Implementation notes

• Pages\**\*.razor[.cs] are added as EmbeddedResource items in both Shared .csproj files with stable SampleSource/{File} LogicalNames. Razor SDK still compiles .razor files normally — EmbeddedResource is additive.
• New ISampleSourceProvider (Shared/SampleSourceProvider.cs in Core.Sample.Shared) reads from pageType.Assembly.GetManifestResourceStream(...), so a Pro app rendering a Core page resolves source from Core, not Pro.
• SourceNav + SourceCode moved into Core.Sample.Shared; Pro inherits via existing ProjectReference. MainLayout.AdditionalLinksType now defaults to typeof(SourceNav) so Core hosts pick it up automatically; Pro's existing override is unchanged.
• SourceCode.razor.cs resolves the page Type by scanning loaded *Sample.Shared assemblies for routable components by name (lazy-cached). Preserves the existing /source-code/{Name} URL contract, the @code-block split, and the ## Filename headings.

Test plan

• dotnet build Core WebApp host — passes (verified locally)
• dotnet build Pro WebApp host — passes (verified locally)
• dotnet build Core Wasm host — passes (verified locally)
• dotnet build Pro Wasm host — passes (verified locally)
• Run Pro WebApp, click Source Code on a sample, verify content loads with no request to docs.geoblazor.com in DevTools Network
• Run Core WebApp, click Source Code on a sample (newly available)
• Pick a page with a .razor.cs code-behind (e.g. CustomPopupContents, StyledGeoJSONLayers, UniqueValueRenderers) and verify both ## Foo.razor and ## Foo.razor.cs sections render
• In the Pro WebApp, navigate to a Core sample and confirm the source resolves from the Core assembly
• Build with /p:UseProjectReferences=true to confirm RazorCopy.dll and the new embedded resources coexist
• Run a MAUI host offline and confirm Source Code still loads