[kanghyunsoon]

Resolves / related issue

Resolves #46.

Summary and validation

• Add a concise contributor guide for the existing manuals, current Sphinx build flow, rendered-page screenshots, writing expectations, and DCO sign-off.
• Add a PR template with explicit screenshot requirements, a tooling-only exception, and a review checklist.
• Add the screenshot review rule to AGENTS.md without copying the contribution guide.

Validated from the root requirements.txt and current Documentation CI workflow with Python 3.12 and Sphinx 9.1.0:

• sphinx-build main _build/main — PASS (1 existing warning)
• sphinx-build user_manual _build/user_manual — PASS (3 existing warnings)
• sphinx-build admin_manual _build/admin_manual — PASS (1 existing warning)
• sphinx-build -W developer_manual _build/developer_manual — PASS, no warnings
• GitHub Markdown rendering of the PR template shows the screenshot section and task-list checkboxes.
• All referenced local manual/workflow paths exist; git diff --check passes.

Screenshots

Not applicable. This PR changes only repository-level Markdown guidance and the GitHub PR template; it does not change any Sphinx-rendered documentation page. The Sphinx builds above confirm the existing manuals still build.

Checklist

• I built the affected documentation locally and reviewed the rendered output (no rendered page changed in this PR).
• I included screenshots of every changed or added rendered documentation page (none changed).
• If screenshots are not applicable, this PR changes no rendered documentation and I explained why.
• I checked that headings, lists, code blocks, images, and links render correctly (GitHub Markdown template and Sphinx builds).
• I did not include personal data, credentials, private documents, or other sensitive information in screenshots (no screenshots).
• I kept the change focused on one documentation concern.
• My commits include DCO sign-off.