Boost: Update foundation page list instructions

[haqadn]

Proposed changes:

• Update instructions under foundation page list input.

Add one URL per line. Only URLs starting with %s will be included. Relative URLs are automatically expanded.

• Reset list to default if empty list is submitted
• Update list limit error message:

You can add only %d foundation page URL.
You can add up to %d foundation page URLs.

• Validate absolute URLs and inform if they belong to a different site.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

None

Does this pull request change what data or activity we track or use?

None

Testing instructions:

None

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• 🔴 Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

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The e2e test report can be found here. Please note that it can take a few minutes after the e2e tests checks are complete for the report to be available.

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🔴 Action required: Please add missing changelog entries for the following projects: projects/plugins/boost

Use the Jetpack CLI tool to generate changelog entries by running the following command: jetpack changelog add.
Guidelines: /docs/writing-a-good-changelog-entry.md

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Follow this PR Review Process:

1. Ensure all required checks appearing at the bottom of this PR are passing.
2. Choose a review path based on your changes:
   • A. Team Review: add the "[Status] Needs Team Review" label
     • For most changes, including minor cross-team impacts.
     • Example: Updating a team-specific component or a small change to a shared library.
   • B. Crew Review: add the "[Status] Needs Review" label
     • For significant changes to core functionality.
     • Example: Major updates to a shared library or complex features.
   • C. Both: Start with Team, then request Crew
     • For complex changes or when you need extra confidence.
     • Example: Refactor affecting multiple systems.
3. Get at least one approval before merging.

Still unsure? Reach out in #jetpack-developers for guidance!

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Boost plugin:

• Next scheduled release: none scheduled.

If you have any questions about the release process, please ask in the #jetpack-releases channel on Slack.