docs(getting-started): [T198] add getting_started.md quick-start tutorial (#297)

New linear FRED-MD -> VAR(4) -> Cholesky IRF -> plot tutorial with runnable
@example blocks (verify_examples OK). Registers a Getting Started nav section
in make.jl and prepends the page to the index @contents block.

Claude-Session: https://claude.ai/code/session_01U961hhdMRdrcbNjmACaVVQ
