Docs: highlight importance of ansible_collections at root dir (#4060)

- change "the `collections/ansible_collections`" to "a
`collections/ansible_collections`", since it doesn't seem to matter
where that directory lies and, at least on my system, the two ones that
already exist are not writable by a normal user by default.
- Highlight that it is not only recommend, but important that the
collections root is named `ansible_collections`. If you put your
collections somewhere else, but follow the guide in every other regard,
you will get a "role not found" error.

docs/getting-started.md

@@ -6,7 +6,7 @@ familiar with the basics of how to use Molecule and what it can offer.
 
 1.  Create a collection
 
-    One of the recommended ways to create a collection is to place it under the `collections/ansible_collections` directory.
+    One of the recommended ways to create a collection is to place it under a `collections/ansible_collections` directory. If you don't put your collection into a directory named `ansible_collections`, _molecule won't be able to find your role_.
 
     ```bash
       ansible-galaxy collection init foo.bar