[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.

[alaudazzi]

From a Slack thread with @mlliarm:

I can provide information on how ecctl can be installed based on its latest published version on Windows and Linux.

For both the method would be like the one mentioned in this KB: https://support.elastic.dev/knowledge/view/f454ec94

Also I'd suggest that also an additional subsection is added on how to build ecctl from source.
so that newer fixes (eg: bug1, bug2) that haven't been packaged in a new release yet are available, using the info from this KB.

CC @eedugon