Automate index creation when enabling Elasticsearch integration

What does this MR do?

Issue #239270 (closed)

Automatically create an empty Elasticsearch index if one doesn't exist before updating Advanced Search settings.

Minor update to the settings UI to alert the user that an empty index will be created if one doesn't exist.

Documentation has been updated to reflect new instructions on how to enable Advanced Search + what the Enable Elasticsearch Indexing checkbox option now does.

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