Add BenchmarkDotNet support to Library template

[Copilot]

Adds BenchmarkDotNet infrastructure to the Library template and a GitHub Actions workflow to track benchmark results over time via github-action-benchmark.

Template changes

• benchmarks/MyLib.Benchmarks/ — new project (targeting net10.0) with sample benchmarks against the library's public API and a Program.fs entry point
• MyLib.slnx — benchmarks project added under a /benchmarks/ solution folder
• Directory.Packages.props — BenchmarkDotNet 0.14.0 added
• .gitignore — BenchmarkDotNet.Artifacts/ excluded

Build system

• New RunBenchmarks FAKE target runs dotnet run --configuration Release across all benchmarks/**/*.??proj
• Depends on DotnetBuild; invocable standalone via ./build.sh RunBenchmarks

CI workflow (.github/workflows/benchmark.yml)

Runs on push to the release branch only, executes RunBenchmarks, then stores JSON results in docs/benchmarks/ on the release branch via github-action-benchmark. Configured with gh-pages-branch: MyReleaseBranch and benchmark-data-dir-path: docs/benchmarks since the project uses the docs/ folder (not a separate gh-pages branch) for GitHub Pages. Includes regression alerting at 200% threshold.

Docs

• README.md documents the RunBenchmarks target alongside other build targets
• docsSrc/index.md includes a "Benchmarks" card linking to the benchmark results page (benchmarks/index.html)

Tests

Integration tests for library projects (MyCoolLib, fsharp-data-sample) assert the benchmark files are generated and that RunBenchmarks builds successfully. Non-library project tests are unaffected.

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[TheAngryByrd]

@copilot as per https://github.com/benchmark-action/github-action-benchmark?tab=readme-ov-file#gh-pages-branch-required, since we use the docs/ folder in the repo, we need to set gh-pages-branch to the default-branch and benchmark-data-dir-path. We should then create a link in the docs/ to that benchmark result.

[Copilot]

@copilot as per https://github.com/benchmark-action/github-action-benchmark?tab=readme-ov-file#gh-pages-branch-required, since we use the docs/ folder in the repo, we need to set `...

Done in commit 1be20d8. The benchmark workflow now sets gh-pages-branch: MyReleaseBranch and benchmark-data-dir-path: docs/benchmarks so results are stored in the docs/ folder alongside the FSDocs-generated site. The workflow also only runs on push (not pull_request) with permissions: contents: write for the auto-push. A "Benchmarks" card linking to benchmarks/index.html was added to docsSrc/index.md.