Improve docs experience for Experience Baseline JTBD: Add my existing Kubernetes cluster

As part of the FY20-Q3 documentation OKR &1678 (closed), this issue pertains to a "job to be done" (JTBD) in GitLab--"Add my existing Kubernetes cluster"--which has been evaluated by UX as part of an Experience Baseline Review.

If you are working on this issue, review the linked OKR and the Experience Baseline Recommendations Handbook page for more detail. Then review the links below, per the OKR instructions.

• UX Evaluation issue - &1380 (closed)
• UX Evaluation video - https://www.youtube.com/watch?v=xAi9u2eqrSk&t=7s
• UX recommendations issue - &1382 (comment 198461475)
• Potential doc work
  • #30577 (closed) (was https://gitlab.com/gitlab-org/gitlab-ce/issues/65407)
  • #30575 (closed) (was https://gitlab.com/gitlab-org/gitlab-ce/issues/65405)
• Product designer - @tauriedavis

Based on the steps in the OKR epic, use the checkboxes to indicate when each step is complete, and use the format below for notes and plans. You are welcome to alternatively link any/all subsections to an additional issue or a Google doc, to work there instead.

• Review existing info
• Review SEO
  • Document and improve SEO
• Consider potential UI doc link
• Review and improve documentation content
  • Test the instructions
  • Plan documentation improvements/additions
  • Ask questions of and share plans with Product Designer
  • Implement doc changes

Plans & Notes

1. Review existing info

Original documentation split creating new Kubernetes cluster in different sections, depending on the type.

Gave the impression that only GKE was supported, even though EKS was possible.

2. Review SEO

It might take some time for the changes to the structure to show up in Google engines. Searches for EKS cluster and GitLab still link to the old page, but will redirect to the relevant section on the new page.

In time, searches for adding and removing clusters in GitLab (regardless of platform) should land in the new page.

3. Consider potential UI doc link

Existing UI had doc link, which needed top be updated for new structure in: !18233 (merged).

4. Review and improve documentation content

Before:

• Creating clusters was split between:
  • Main page: https://docs.gitlab.com/ee/user/project/clusters/
  • EKS page: https://docs.gitlab.com/12.4/ee/user/project/clusters/eks_and_gitlab/
• Many creation concerns were buried in the main documentation under configuration. For example: https://docs.gitlab.com/12.4/ee/user/project/clusters/index.html#access-controls

After:

• Information about adding/creating and removing clusters is located on a single target page: https://docs.gitlab.com/ee/user/project/clusters/add_remove_clusters.html.
• Prerequisite information is at the top: https://docs.gitlab.com/ee/user/project/clusters/add_remove_clusters.html#access-controls, not buried deep in the bottom.
• Main cluster page "does less work": https://docs.gitlab.com/ee/user/project/clusters/.

d. Implement doc changes

Docs changes implemented in: !18233 (merged).