Update Build/Install Indexer section to clarify what is necessary in Omnibus installs

What does this MR do?

This MR is a bit redundant, but I feel it is necessary, at least temporarily. Currently we have an open issue discussing the long term formatting of allowing a customer to only see instructions pertaining to their install by collapsing the instructions that are not needed. Currently, for many of our product features, Omnibus and Source instructions are side by side.

This Elasticsearch page is an example of this. Currently, all the headers are the same size, making it really easy for someone to mistake the instructions to the Indexer installation section. We have removed the need to do this from Omnibus installs, but the way it is presented and its distance from the note in the Omnibus section saying an indexer is already included for Omnibus installs has caused customers to open Support tickets not realizing they did not have to build an Indexer.

I changed the header sizes for the subsections of the Source install and also added a note reminding users they do not need to do that step for Omnibus installs.

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group label, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add GitLab's version history note(s).
• Add the product tier badge.
• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
  • Add Technical Writing and docs:: workflow label.
  • Add docs-only when the only files changed are under doc/*.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.