[Docs] docs/index.rst: replace EE overview paragraph with a link to docs.ansible.com EE page

[Andersson007]

• Replace the content of "What are execution environments" with a reference to the docs.ansible.com guide.
• The links to ansible control nodes term explanation was removed in order to avoid inline links to things that users should be already aware of

[Andersson007]

i'm not sure if codecov failures relate to the changes, so ready for review

[Andersson007]

@Akasurde thanks for reviewing and merging!