Helm chart install page: start structural revisions

What does this MR do?

Now that !5030 (merged) is merged, let's start moving the subheadings into a more logical order. This MR modifies:

• Move ## Configuring GitLab Runner using the Helm Chart above the Install subheading, as it's essentially a long and complex prerequisite
• Handle subheadings of ## Configuring GitLab Runner using the Helm Chart:
  • blend child subheading ### Required configuration (child) back into it
  • move child subheading ### Additional configuration from H3 to H2, and bump those to the bottom of the page to get a cleaner flow through the major CRUD steps
  • because Additional configuration changed a H2, update the heading levels for the subheadings underneath
• Move ## Check available GitLab Runner Helm Chart versions from an H2 further down the page, to a H3 nested underneath Installing, because the Installing subheading ends with a description of selecting a chart version.
• Move ## Uninstalling GitLab Runner using the Helm Chart above all the additional configuration options, to keep the major parts of CRUD together.

Related issues

• Follow-up to Address line-level findings in Kubernetes insta... (!5030 - merged)
• Related to technical-writing#1075

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