Docs: Add FIPS AI Gateway image to install docs and FIPS compliance page

What does this MR do?

Add FIPS-compliant AI Gateway image documentation to the public install page and the FIPS compliance page.

The FIPS AI Gateway image (model-gateway/self-hosted-fips) has been building and publishing automatically since 18.10 (#560241 (closed)). Internal delivery docs were updated in ai-assist!5168, but the public-facing install page and FIPS compliance page were not updated.

This MR adds:

• A "FIPS-compliant images" subsection to doc/install/install_ai_gateway.md with registry links, CMVP certificate reference, and container swap instructions
• An "AI Gateway" entry in doc/development/fips_gitlab.md (FIPS compliance page)

Related issues

• #560241 (closed)
• gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#2124 (closed) (Part 3 of this issue -- public docs -- was not addressed by !5168 (merged))

Author's checklist

• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Not adding a new page, so availability details are not needed.
• Request review based on page metadata (AI-powered / AI Framework).

AI-Generated Content Disclosure: This MR contains documentation drafted with assistance from Claude Code. The output has been reviewed for correctness and validated against the existing FIPS image state on Docker Hub and GitLab Container Registry.