docs: reorganize "getting started" + cleanups old docs

Our README is currently pretty cluttered:
- Part of the README duplicates docs/getting_started/getting-started.md
-> Remove duplicates and extract useful info into the getting-started.md

- General info on Ansible environment troubleshooting
-> remove most of it as it's not specific to Kubespray, move to
docs/ansible/ansible.md
-> split inventory-related stuff of ansible.md into it's own file. This
should host documentation on how to manages Kubespray inventories in the
future.

ansible.md:
- remove the list of "Unused" variables, as:
  1. It's not accurate
  2. What matters is where users should put their variables