docs(installing/onboarding): create section about security/privacy

[HonkingGoose]

Changes

• Create short section pointing to the Security and Privacy docs on the Onboarding and Installing Renovate page

Context

We're getting more questions about the security and privacy implications of installing Renovate into a repository or organization lately. For example:

• Renovate requires app write access for code? #29362
• developer.mend.io and private repos: security considerations #29327

I guess many users do not find the Security and Permissions page that explains the how and why. The page is a bit hidden in the All Other collapsed sidebar entry.

Maybe adding a short reference in the onboarding/installing part of the docs helps users actually read the page. But that supposes that users will read the installation instructions before installing the bot. 🙃

Maybe we should also move the Security and Permissions page so it has its own entry? That way you can see it when just "browsing" the sidebar.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 37.382.5 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 37.382.5

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