Advanced search and AWS OpenSearch config docs update

Background

Customers are asking questions that show the docs are not clear enough for them to figure out that: a) this configuration is possible, b) why it is "odd" compared to most AWS security configurations.

1. Even experienced AWS operators may not have previously used the uniquely one-sided service-linked roles where they do not have to configure GitLab with an AWS role ARN to complete the permissions loop - this update clarifies that. The linked AWS docs literally say "A service-linked role is a unique type of IAM role that is linked directly to OpenSearch Service."
2. With the lack of a screenshot in the docs and the lack of a field for an AWS Role ARN - customers have to eventually stumble upon https://docs.aws.amazon.com/opensearch-service/latest/developerguide/slr.html to understand that this "apparently" one sided configuration will work.

What does this MR do?

This MR makes documentation updates for Advanced search:

• Restructure where the configuration information for AWS OpenSearch lives
• Clearly defines the three types of authentication for AWS OpenSearch and how that maps to the Admin UI settings for Advanced search
• Adds troubleshooting errors gathered from customer support tickets and manual testing

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product tier badge under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.