[Chore] Recommend public node address(es) in the introduction

Currently the tutorial's introduction suggests using a shared or public node, but none is recommended.

Right after that it also explains how to configure the client to remember the node address, which will cause problems (much) later in the tutorial if someone skips this part or gets it wrong.

This adds a public node recommendation (any from https://teztnets.xyz/) and slightly rewords this part of the tutorial.