tt cfg dump

**Tracked in**: #3168 
**Related dev. issue(s):** https://github.com/tarantool/tt/issues/135

**Product:** Tarantool CLI
**Since:** ??
**Audience/target:** dev, admin
**Root document:** tt cli docs
**SME:** @ LeonidVas 

# Details

Add docs on the ``tt cfg`` module

# Definition of done

- [ ] a new reference page about ``tt cfg`` is added
