Merge the Gitaly docs with the ones hosted in GitLab

What does this MR do?

Bring all user facing content of https://gitlab.com/gitlab-org/gitaly into GitLab.

• Copy content to https://docs.gitlab.com/ee/administration/gitaly
• Link to it from https://gitlab.com/gitlab-org/gitaly/blob/master/doc/configuration/README.md explaining the new location
• Add reference to https://gitlab.com/gitlab-org/gitaly/blob/master/config.toml.example
• Copy content of gitaly-debug https://gitlab.com/gitlab-org/gitaly/blob/master/cmd/gitaly-debug/README.md (under troubleshooting section https://docs.gitlab.com/ee/administration/gitaly/#troubleshooting-gitaly-in-production)
• Update main README with the information and maybe copy the architecture diagram to the docs

Related issues

gitaly#1386 (closed)

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.