Fix spelling and formatting errors

What does this MR do?

I spotted these Kramdown warnings while doing a full build, and while I was here, fixed a few spelling issues.

      create  [0.04s]  public/charts/development/preparation/index.html
      create  [0.11s]  public/charts/development/changelog-manager.html

kramdown warning(s) for <Nanoc::Core::CompilationItemRepView item.identifier=/charts/charts/gitlab/mailroom/index.md name=default>
      create  [0.14s]  public/charts/charts/gitlab/migrations/index.html
  No link definition for link ID '' found on line 1
  No link definition for link ID '' found on line 1
      create  [0.14s]  public/charts/charts/gitlab/gitaly/index.html
  No link definition for link ID '' found on line 1
      create  [0.13s]  public/charts/charts/gitlab/sidekiq/index.html

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)
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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:

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• Add the product tier badge.
• 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

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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.

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• 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.
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  • Add twdoing when starting work on the MR.
  • Add twfinished if Technical Writing team work on the MR is complete but it remains open.

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