feat(docs): add llms.txt feed and optional description generation for better LLM routing

[benjaminach]

Summary

This PR introduces a generated llms.txt feed for doc.scalingo.com and improves LLM routing quality by adding optional, high-signal page descriptions.

Changes

• Added src/llms.txt (Jekyll-generated) to expose a machine-readable list of documentation pages.
• Updated _config.yml to include llms.txt in the generated site.
• Filtered out non-target content from the feed (e.g. changelog entries, directory/index-like pages, explicitly excluded pages).
• Added a new Codex skill: .codex/skills/scalingo-generate-description/SKILL.md.
• Added its OpenAI agent config: .codex/skills/scalingo-generate-description/agents/openai.yaml.
• Added/updated description front matter on selected user-management pages.
• Standardized description formatting with double-quoted YAML values.

Why

description fields are optional, but they significantly help LLMs understand page intent and route queries to the right documentation page when scanning llms.txt.

Impact

• New machine-readable endpoint: /llms.txt
• Better semantic retrieval/routing for doc-focused LLM workflows
• No expected user-facing behavior changes in normal site navigation

Validation

• Verified Jekyll generation includes llms.txt.
• Checked feed formatting and URL output.
• Checked YAML front matter validity on updated pages.