docs: require security review before submitting files

[developer0hye]

Summary

Require a security review before contributors submit files to improve office2pdf. Organizational files require internal discussion with the responsible security team or authorized reviewer and approval for public sharing.

The new contributor guide explains sharing permissions, removal of sensitive content, submitter responsibility, and a disclaimer of liability to the extent permitted by applicable law. The README and issue/PR templates surface the policy before files are submitted.

Related issue

Requested by the repository owner; no existing matching issue found.

Testing

• Verified Markdown link targets and issue template front matter.
• git diff --check passed.
• Required read-only documentation freshness audit: PASS.
• Runtime tests skipped under repository policy because only documentation and submission templates change.

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: Documentation and GitHub templates only.