DOC: add security reporting guidance and issue templates

[aryansk]

Summary

• update the operator documentation with the current private security-reporting process
• add a security issue template that redirects reports to a private GitHub Security Advisory or security@jupyter.org
• add a security note to the bug-report template

Fixes #250

Testing

• git diff --check
• The local pre-commit, doc8, and Hatch toolchains were not installed, so the repository's full documentation lint could not be run locally.

This change was prepared with AI assistance and reviewed against the current Jupyter security policy.

[krassowski]

Iissue templates are stored in https://github.com/jupyter-server/.github/blob/main/.github/ISSUE_TEMPLATE/bug_report.md IMO no point in overwriting it here with a different template. Also I don't think including the proposed security note in this form a good idea - users will just include it as-is an it will show up in all issues.

The update of email address in security.rst is warranted and the link to jupyter.org/security is a good idea. Otherwise I don't believe any other changes are needed in this repo

[aryansk]

Thanks for the review. I removed both repository-local issue-template files as suggested, including the public bug-report security note. The branch now keeps only the security documentation update: private GitHub Security Advisory reporting, the current security@jupyter.org address, and the Jupyter security-policy link. This leaves issue-template ownership with the organization-level repository. Please let me know if you would like the documentation wording narrowed further.