docs(rails): add rail installation guidance

[Pouyanpi]

Description

Build installation plans from rail manifests and expose them through
nemoguardrails rails requirements for selected rails or a configuration. The
command supports human-readable, requirements-file, and JSON output and never
installs packages or reads environment-variable values.

Generate a central requirements reference and an installation block on every
built-in rail catalog page. Pre-commit and tests reject stale generated content.

Generated documentation:

• docs/configure-rails/guardrail-catalog/rail-installation-requirements.mdx
  is fully auto-generated.
• Blocks between {/* BEGIN GENERATED RAIL REQUIREMENTS */} and
  {/* END GENERATED RAIL REQUIREMENTS */} in
  docs/configure-rails/guardrail-catalog/**/*.mdx are auto-generated. The rest of
  each catalog page remains hand-authored.

For future manifest changes, run make generate-rail-requirements and commit the
result. Documentation preview and publication workflows should run
make check-rail-requirements before the Fern build so stale pages cannot be
published. The published site should consume committed MDX rather than generate
requirements at runtime.

Related Issue(s)

• Fixes #
• Issue assignee: @

Verification

• Full make test: 5,951 passed, 178 skipped.
• Pre-commit and uv lock --check passed.
• Local Fern preview rendered the generated catalog pages.

AI Assistance

• No AI tools were used.
• AI tools were used; a human reviewed and can explain every change (tool: Codex).

Checklist

• I've read the CONTRIBUTING guidelines.
• This PR links to a triaged issue assigned to me.
• My PR title follows the project commit convention.
• I've updated the documentation and tests.
• I've noted checks that require credentials.
• I did not update generated changelog files manually.
• @mentions of the person or team responsible for reviewing proposed changes.