fix links referencing multi-session feature; update userbutton component reference

[alexisintech]

🔎 Previews:

• https://clerk.com/docs/pr/2010

What does this solve?

• Links referencing our multi-session feature were leading users to the /custom-flow guide. Custom flows are advanced and not recommended as a first solution.

What changed?

• Updates links to point to the dedicated section for our multi-session feature.
• Adds copy (for supported SDKs) about recommending users to wrap their app with our <MultisessionAppSupport /> component
• Adds copy (for supported SDKs) to bring awareness to the afterMultiSessionSingleSignOutUrl prop
• Updates the <MultisessionAppSupport /> document to include Next.js App Router example, and updates the other examples to be consistent with their appropriate quickstarts
• Updates the <UserButton /> component reference
  • Updates introductory paragraph that explains what the feature does to be more comprehensive
  • Updates the image as it had a typo, and also was clipped at the top.
  • Increases its rank so in search, it appears above the "Add custom items and links to the component" doc
  • Updates code examples to be consistent across frameworks
  • Adds the <If> component to wrap around the "Usage with JavaScript" section in order to hide it unless the active SDK is JavaScript

Checklist

• I have clicked on "Files changed" and performed a thorough self-review
• I have added the "deploy-preview" label and added the preview link(s) to this PR description
• All existing checks pass

[github-actions]

Hey, here’s your docs preview: https://clerk.com/docs/pr/2010