ci: migrate generated docs to GitHub Pages with PR preview

[nh13]

Problem

The current CI workflow auto-commits "Generate docs files" to PR branches, which causes two issues:

1. Rebase conflicts on every merge: docs/tools/index.md embeds a version+commit hash (e.g. 0.2.1-b54e45d), so every open PR's generated docs conflict with main after any merge. This forces manual rebasing of all open PRs.

2. Lost GitHub status checks: Commits pushed by GITHUB_TOKEN don't re-trigger workflows (by design, to prevent infinite loops). So the final commit on every PR branch is the unchecked "Generate docs files" commit, and the test/lint status checks are no longer associated with the HEAD commit.

Proposed Solution

Migrate generated docs from the main branch to a gh-pages branch, following the pattern used by fgbio.

Key changes

• Create a gh-pages branch with Jekyll (modernist theme) to host docs at https://fulcrumgenomics.github.io/fgsv/
• PR previews: Use rossjrw/pr-preview-action to deploy doc previews for each PR (same as fgbio)
• Main push: Auto-generate and deploy docs to gh-pages on merge to main
• Independent test job: Remove needs: docs dependency so tests run independently and status checks are always on the final PR commit
• No auto-commits to PR branches: Eliminates rebase conflicts and lost status checks entirely
• Remove generated docs from main: docs/tools/*.md and docs/04_Metrics.md move to gh-pages only; add to .gitignore
• Keep hand-written docs on main: docs/01_Introduction.md, docs/02_Installation.md, docs/03_Contributing.md stay as source-controlled content, copied to gh-pages during CI builds

What stays the same

• Doc generation logic (BuildToolDocs, FgSvMetricsDoclet) is unchanged
• Build scripts (src/scripts/build_tool_docs.sh, src/scripts/build_metric_docs.sh) remain for local dev use
• Unit tests continue to run on push and PR

Additional improvements

• Update actions/checkout from v2 to v4
• Remove GPG signing infrastructure (no longer needed without auto-commits)
• Landing page on GitHub Pages with badges, install instructions, and links to tool/metric docs

References

• fgbio's implementation: workflow, composite action, live site
• rossjrw/pr-preview-action