docs: improve installation and overviews

[itaysk]

Description

• improved installation doc
• improve overview pages
• rename "CLI" section to "Docs"

Related issues

• Close Is the trivy homebrew formula official from aquasecurity? #3388
• Close Installing trivy via homebrew formula aquasecurity/trivy/trivy fails with "cannot be installed from bottle" #3387

Related PRs

Remove this section if you don't have related PRs.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).