Preserve anchors in docs redirections

In gitlab-org/gitlab!152906 (merged) @mrincon raised the below issue:

Pages does not preserve anchor links when redirecting based on entries in the _redirect file.

@mrincon started a discussion:

> **Issue:** Not related to this MR, but the redirects don't preserve the anchors:
> 
> https://docs.gitlab.com/ee/ci/pipelines/cicd_minutes.html#cost-factor
> 
> Sends me to:
> 
> https://docs.gitlab.com/ee/ci/pipelines/compute_minutes.html