Miscellaneous cleanup

[tdecroyere]

Summary

Collect small, self-contained cleanup work before the next larger Elemental architecture iteration.

Current changes:

• rewrite the root README around Elemental's modern GPU programming mission;
• clarify the project's design principles and modern-only direction;
• replace the outdated feature/version description with the current capabilities and sample progression;
• document future research directions without presenting them as implemented APIs;
• update the MIT license copyright year to 2026;
• replace the stale samples index and add focused README documentation for every current Elemental sample, demo and ElementalTools sample.

Additional small cleanups may be added to this PR as they are identified during the cleanup pass.

Sample documentation

The sample documentation now distinguishes between:

• progressive samples/Elemental examples focused on the public runtime API;
• larger samples/Demos experiments used as R&D/stress laboratories;
• samples/ElementalTools command-line examples for offline shader and asset processing.

The READMEs describe the actual current code paths, controls and limitations. In particular, AITraining is documented as a CPU baseline rather than an already GPU-accelerated ML sample, and the sample .scene / .texture formats are explicitly not presented as stable public Elemental formats.

Scope

This PR is intentionally limited to cleanup and maintenance work. It should not opportunistically redesign major Elemental APIs or absorb the upcoming GPU virtual-memory architecture work.

Validation

Documentation-only so far. The sample documentation was checked against the current source files and CMake targets on the PR branch. Further validation notes will be added if code changes are included later.