OKR: Update external wiki UI text and docs

What does this MR do?

• Updates UI text for external wiki integration:
  • The UI says "Replaces the link to the internal wiki with a link to an external wiki" but this is not true. The external wiki link does not replace the internal link.
  • Add help text linking to the docs.
• Docs added and updated: Docs were missing - there was just an (outdated) mention in the integration index. This was updated along with new dedicated docs.
• Closes #300596 (closed)
• Closes #27656 (closed)
• For technical-writing#352 (closed)

Screenshots (strongly suggested)

BEFORE	AFTER

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • [-] I have not included a changelog entry because _____.
• 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