[Doc] Installing section only containing macOS

[gemaliano]

Installing section only containing macOS -> https://www.elastic.co/guide/en/ecctl/current/ecctl-installing.html
Installing section missing Linux OS.

[alaudazzi]

@gemaliano
Thank you for providing the required input. We are happy to assist you with the doc changes and the review.

[mlliarm]

Hello @alaudazzi, I'd love to help with the addition of the installation steps for DEB and RPM GNU/Linux systems.

I can prepare a draft and share it here, or elsewhere.