Fixes for the getting started documentation

[Olifantti]

The getting stared docs might be slightly out of date, I had trouble installing everything correctly and stumbled with the fetching instructions

The folders cloned parallel to lang-xxx were different

This will automatically clone the folders 
giella-shared and giella-core parallel to lang-XXX. 

Mine were
giella-core shared-mul shared-smi

There wasn't anything about setting up SSH keys before cloning, but these exist

• Creating a private key - GT
• Passwordless login by using ssh - GT
• Generating a new SSH key and adding it to the ssh-agent - GitHub Docs
  It was really simple once I got it done but very confusing at first without knowing they exist 😆