docs(config): document backend.signoff_commits option (DCMS-442)

[sempostma]

Summary

Documents the signoff_commits backend config option on the Configuration Options page. This option is fully implemented in packages/decap-cms-core/src/lib/formatters.ts (commitMessageFormatter, decap-cms repo) — it appends a git-style Signed-off-by: {name} <{email}> trailer to every generated commit message when enabled — but was never documented on decapcms.org.

Adds a new "Sign Off Commits" subsection under Backend > Commit Message Templates, documenting:

• type (boolean) and default (false)
• the appended trailer format
• the console-warning fallback when author name/email is unavailable

Fixes DCMS-442. Issue: /Users/sempostma/Documents/personal/Projects/decap-cms/Issues/DCMS-442 - backend.signoff_commits config option implemented (adds Signed-off-by trailer) but not documented on decapcms.org Configuration Options page.md

Test plan

• Diffed against sibling commit_messages section for style/formatting consistency
• hugo not installed locally; repo's own lint script only covers layouts/eslint/scss, not markdown content — no applicable lint/build gate found
• Docs-only change, no code touched