Fix edit this page URL for charts docs

Currently, the newly published charts documentation uses a fallback URL for "edit this page" button, which is invalid.

This manifests in lots of broken links in the external link checker: https://gitlab.com/gitlab-com/gitlab-docs/-/jobs/168945947.

This adds the logic to generate a correct URL, do the link checker output is less noisy and people using the charts documentation don't get a 404 when they attempt to follow the edit this page link.

Example (click edit this page button):

• Before: https://docs.gitlab.com/charts/installation/version_mappings.html
• After: http://create-correct-edit-this-page-link-for-charts.178.62.207.141.xip.io/charts/installation/version_mappings.html