Add docs issue form and tighten issue triage response targets

[monthop-gmail]

Motivation

• Provide a dedicated structured issue form for documentation work so the triage taxonomy includes docs alongside bug, question, governance, and architecture.
• Make issue triage actionable by enforcing supported labels and adding response-time targets for first response and triage decisions.
• Align the roadmap's issue-operations section with the updated triage label set so documentation issues map cleanly into planning.

Description

• Added new issue templates under .github/ISSUE_TEMPLATE/: docs-improvement.yml, bug-report.yml, governance-change.yml, question.yml, and a config.yml to control issue form behavior.
• Removed the deprecated .github/ISSUE_TEMPLATE/issue_template.md placeholder file.
• Created docs/issue-triage.md with intake rules, supported label list, prioritization matrix, a response SLA (First response and Triage decision targets), and weekly cadence guidance.
• Updated docs/roadmap.md to reference the full label set (including docs) and to align cross-cutting issue operations with the triage guide.

Testing

• Inspected and validated new template contents and formatting using file display commands such as nl -ba .github/ISSUE_TEMPLATE/docs-improvement.yml and nl -ba docs/issue-triage.md, which showed the expected fields and sections.
• Confirmed docs/roadmap.md contains the updated Issue Operations section referencing the expanded label set and SLAs.
• Performed repository-level validation of changed files and created the PR; all configuration/documentation changes validated as expected (documentation-only changes).

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Codex Task