Sync README, tutorial_2, tutorial_3 with published docs

Mirror agents-langgraph.mdx, multi-agent-mcp-targeting.mdx, and
ai-experiments-roi.mdx from ld-docs-private. Rewrite asset paths to
the local screenshots/ directory and absolute-ize internal doc links
to https://launchdarkly.com/docs/... so the files render correctly
on GitHub.

Author: sattensil