docs(config): add llm request tagging guide

[SleepySML]

Summary

Documents the LLM request tagging feature shipped in MR #3775 (EPMCDME-12602). CodeMie now injects X-CodeMie-Version and X-CodeMie-Project HTTP headers on outbound LLM calls for supported provider paths.

Changes

• Added docs/admin/configuration/observability/request-tagging.md — new dedicated page covering injected headers, provider coverage table, purpose/use cases, configuration, and verification steps
• Updated sidebars.ts — added request-tagging to the Observability category
• Added faq/what-http-headers-does-codemie-add-to-outbound-llm-requests.md — FAQ entry for header names and values
• Added faq/which-llm-providers-receive-codemie-version-and-project-headers.md — FAQ entry for provider coverage
• Fixed scripts/secrets-check.sh — quoted $CONTAINER_ENGINE and added Windows (MSYS/Git Bash) path conversion so Gitleaks runs correctly on Windows with Podman

Testing

• Tested locally with npm start
• All pages render correctly
• Images display properly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• No MDX compilation errors
• No raw angle brackets (<text> must be `<text>`)
• Sidebar references document IDs (not filenames)
• Images stored locally next to content (not in static/img/)
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation

Additional Notes

Provider coverage documented: Azure OpenAI/DIAL ✅, Google Vertex AI ✅, Anthropic direct ✅, AWS Bedrock ❌ (no custom HTTP headers), LiteLLM proxy ❌ (uses x-litellm-tags instead).