feat: migrate legacy docs (#1568)

...s/llamaindex/modules/query_engines/metadata_filtering.mdx

.../llamaindex/modules/query_engines/router_query_engine.mdx

.../content/docs/llamaindex/modules/response_synthesizer.mdx

apps/next/src/content/docs/llamaindex/modules/retriever.mdx

apps/next/src/content/docs/llamaindex/modules/workflows.mdx

...ext/src/content/docs/llamaindex/recipes/cost-analysis.mdx

apps/next/src/content/docs/llamaindex/recipes/meta.json

apps/next/src/content/docs/llamaindex/what-is-llamaindex.mdx