Before this commit some links lead to a 404 page:

* Fix relative links
  * `handbook/*` ➡ `/handbook/*`
* `infrastructure` moved to `engineering/`
  * `/handbook/infrastructure/*` ➡ `/handbook/engineering/infrastructure/*`
* `infrastructure/monitoring` moved to `engineering/`
  * `/handbook/infrastructure/monitoring/*` ➡ `/handbook/engineering/monitoring/*`
* `infrastructure/gitaly` moved to `engineering/dev-backend`
  * `/handbook/infrastructure/gitaly/*` ➡ `/handbook/engineering/dev-backend/gitaly/*`