docs: modernize README for the Library package collection

[Olof-Lagerkvist]

The root README described only LTRData.Extensions, leaving visitors without an overview of the 16 library projects now in the repository.

This replaces that introduction with:

• A complete package/component table linked to the project files, plus package installation guidance.
• The shared target framework matrix and all three library-specific exceptions.
• Platform and runtime notes for native helpers, CIM, thermal sensors, ASP.NET and SkiaSharp.
• An overview of the expression/plotting/rendering boundaries and links to the existing specifications, samples, migration notes and retained design history.
• Build/test commands reflecting CI, Release package output guidance, and links to the local NuGet workflow.
• Context for the separate LTRLib repository and Geodesy's MightyLittleGeodesy provenance.

Only README.md changes. Full GitHub links also work when this README is embedded in NuGet packages.

Validation: inspected master at 5c0c4e9, the complete tree, recent history, every project file, shared build properties, CI, detailed math/package documentation and representative implementations. Checked all 16 library projects are represented and every repository file link exists. Read back the committed README and confirmed it matches the reviewed text. Build/test commands were checked against configuration, not executed in this session.

Separate finding for owner review: Directory.Build.props declares MIT package metadata, but LTRData.Extensions.Native/NativeLib.cs contains an Arsenal Consulting AGPLv3 header and refers to LICENSE.txt, which is absent from the inspected tree. This PR makes no licensing changes or blanket MIT claim; the discrepancy needs separate clarification.