docs(config): document image allow-list for llm output

[KyryloKorotych]

Summary

Documents the image domain allow-list delivered in EPMCDME-12950 (codemie!3976, codemie-ui!1616) — the security control that gates every image rendered from LLM/assistant output against a per-deployment hostname list, closing a prompt-injection exfiltration channel.

Adds a full guide under Security & Compliance and documents the allowedImageDomains customer config item in the configuration reference.

Changes

• Added docs/admin/security/llm-output-image-allow-list.md — threat model, scope of the gate, the 9-condition decision order, entry format, configuration, rollout and verification steps
• Updated customer-feature-configuration.md — new CONTENT SECURITY group with the allowedImageDomains row in the component overview table, a dedicated section under Component Categories, and an entry in the full configuration example
• Added two FAQ entries: why assistant images are not displayed (end user), and how to allow a specific domain (administrator)
• Registered the new page in sidebars.ts and cross-linked it from the Security & Compliance index; freed sidebar_position: 3 by moving Network Policies to 4
• Added exfiltration and srcset to the cspell dictionary

Testing

• Tested locally with npm start
• All pages render correctly
• Images display properly — no images added; none referenced
• Internal links work — verified by npm run build (zero broken links)
• Sidebar navigation works — new entry resolves during build

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/) — not applicable, no images added
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation — Gitleaks clean

Additional Notes

Verification performed: npm run check and npm run build both exit 0, with zero broken links reported. A visual pass with npm start was not run — the checkboxes above reflect only what was actually verified.

Source-of-truth discrepancy worth flagging to the reviewer: the description on codemie-ui!1616 documents configuration via the VITE_ALLOWED_IMAGE_DOMAINS frontend env var, but that approach is absent from the merged diff — there are no .env, config.js, or deploy-templates changes. The implementation reads appInfoStore.getAllowedImageDomains(), sourced from GET /v1/config → allowedImageDomains, which matches codemie!3976. This documentation follows the code, not the !1616 description — that description should be updated so it no longer contradicts both.

Implementation details documented here that appear in neither MR description, taken from the diff:

• The gate is a DOMPurify hook on a dedicated sanitizer instance, not a marked renderer override — so raw HTML in model output is gated identically to markdown image syntax
• Four attributes are inspected: src, srcset, poster, background; srcset candidates are checked individually
• A blocked <source> element is removed rather than badged, letting the <img> fallback apply; on other elements only the offending attribute is stripped
• Customer config is fetched once per browser tab, so users must reload after the allow-list is changed — this is called out in the rollout steps