Redirect to documentation pages URL when configuration option is set

What does this MR do?

Contributes to issue: #32328 (closed)

Previous MR: !42702 (merged)

Builds URL and redirects to documentation website when Documentation pages URL option in Admin Area is populated.

Fallbacks to default /help rendering when the option is not set.

How to test it locally

1. Fetch 32328_use_documentation_url_to_configure_path_to_help_pages branch
2. Apply migrations if there are any / restart GDK
3. Enable feature flag :help_page_documentation_redirect
4. Go to Admin area -> Settings -> Preferences
5. Expand Help page section
6. Fill Documentation pages URL field with https://docs.gitlab.com
7. Go to http://localhost:3000/help/administration/index.md (or any other documentation page)

Expected result

You will be redirected to https://docs.gitlab.com/ee/administration/index.html.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team