Fix documentation and improve default http client

[jetelain]

No description provided.

[Copilot]

Pull request overview

This PR updates library documentation around rendering APIs, centralizes default HttpClient creation for DEM downloads, and adds a demo application intended to exercise README examples against the current APIs.

Changes:

• Corrects README examples for PDF rendering and topographic map setup.
• Extracts DEM HTTP client creation into a shared helper and reuses it from database entry points.
• Adds a DemoApp project to the solution to compile/run documentation-style samples.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
Pmad.Drawing/README.md	Updates the PDF example to use the current Render.ToPdf overload.
Pmad.Cartography.sln	Adds DemoApp to the solution and updates solution configuration metadata.
Pmad.Cartography.Drawing/README.md	Refreshes topographic rendering/PDF examples to match current APIs.
MapToolkit/Databases/WellKnownDatabases.cs	Switches well-known DEM storages to the shared HTTP client helper.
MapToolkit/Databases/HttpClientHelper.cs	Introduces centralized HttpClient creation with the default User-Agent.
MapToolkit/Databases/DemHttpStorage.cs	Adds XML documentation and routes the Uri constructor through the helper.
DemoApp/Program.cs	Adds an executable sample covering README scenarios across the packages.
DemoApp/DemoApp.csproj	Defines the new .NET 8 demo application project and references.

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