Add dedicated guide pages for two AI coding tools that were missing
from the documentation (closes#744):
- docs/guides/antigravity.mdx — skills install, semantic matching,
Manager view parallelism, MCP alternative, CLAUDE.md compat
- docs/guides/copilot-cli.mdx — skills install, slash commands,
/skills management, agent mode, MCP alternative
Also updates quickstart.mdx and prompting.mdx to mention both tools
alongside the existing agent list, and adds both pages to the docs
navigation in docs.json.