Add architecture, infrastructure, and troubleshooting docs pages

What does this MR do and why?

Copies over more docs pages from gitlab-docs and updates content to reflect the move to Hugo.

Originals:

• https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/architecture.md?ref_type=heads
• https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/infrastructure.md?ref_type=heads
• https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/troubleshooting.md?ref_type=heads

This is one of many MRs for #42 (closed).

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.