docs: improve stabilityDays config option documentation

[HonkingGoose]

Changes:

• Explain that stabilityDays only cares about the time since the release of that specific version not about whether the project as a whole has not released a new version in x days
• Explain that you should set up a custom schedule for fast-releasing projects to slow down the barrage of PRs
• Cover one usecase for stabilityDays: prevent holding broken npm packages (they can be unpublished with 72 hours)

Context:

Based on the confusion from a Renovate user on how stablityDays works.
See discussion #10419.

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 tick one)

I have verified these changes via:

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

[HonkingGoose]

Maybe it's better to put the npm usecase in its own section with its own heading. I've also added a configuration example to the npm usecase explanation.

[renovate-release]

🎉 This PR is included in version 25.41.1 🎉

The release is available on:

• GitHub release
• 25.41.1

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