Summary

This PR replaces the hand-maintained API reference flow with DocFX so the public API docs are generated from XML documentation and stay aligned with the repo's current surface area.

Highlights

DocFX workflow

• Add docfx.json and toc.yml to build the docs site repeatably.
• Generate API reference output from the public packages' XML docs.
• Keep the generated site under the existing Docs/API documentation structure.

Repo integration

• Enable XML documentation output for the public packages that feed the reference.
• Add a docs job to pr-check.yml and include it in validation-summary.
• Document the DocFX build command in README.md.
• Add /_site to .gitignore.

Cleanup

• Fix XML doc warnings in IMutationEngine and MutationExecutor so the docs build stays clean.
• Update Docs/API/Redis.md to point at the new reference entry page.
• Replace the old hand-authored Docs/API/API-Reference.md with Docs/API/Reference.md.

Why

The previous API reference path was manual and easy to drift. DocFX gives us a repeatable generated reference from XML docs and keeps the output aligned with the existing Docs/API layout.

Checks

• dotnet build ModularityKit.Mutator.slnx -c Release
• git diff --check
• docfx docfx.json in CI

Closes

• Closes [Task]: Add API reference publishing from XML docs #52