Docs: Merge EE doc/analytics to CE

What does this MR do?

Ported to EE in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11830.

This aligns the CE and EE analytics docs, as part of the effort to combine docs into a single codebase (&199 (closed)).

This MR:

• Brings in the EE-only doc/analytics directory and the two files in it.
• Changes the relative links to absolute links.
• Renames the readme.md to index.md to pass the lint test, and decreases the readme count in the lint test by one to reflect that.

When both the EE Port and this MR are merged, the doc/analytics docs should hopefully be aligned between CE and EE.

Then, please merge: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11874, and the issue will be closed.

Related issues

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/60836

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the ~Documentation label.

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.

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 and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.