Add frontmatter `mapped_pages` validation

[reakaleek]

Describe the bug

When a user inputs a URL that does not start with https://www.elastic.co/guide the version picker does not show a current version.

Expected behavior

The docs-builder command should fail and only allow URLs starting with https://www.elastic.co/guide, the error message should be clear about this and include steps on how to fix it.

Steps to reproduce

Add a link to https://www.elastic.co/docs/get-started/deployment-options into the frontmatter mapped_pages attribute and open the page where you configured it.

Tooling

• docs-builder
• migration tooling
• I'm not sure