Add template partial for legacy path

What does this MR do and why?

• Adds a template partial to find the legacy URL for the page. We'll need this for the Versions menu so that links to pre-Hugo versions will still work.
• Refactors the edit-link shortcode so that we can share the page-source-project function between these two templates.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/doc/setup.md.
2. Visit a few pages from each source project, and verify:

• The "Edit page" links in the footer still work
• The gitlab-docs-legacy-path metatag contains the path to this page that works on the Nanoc site. Pages from the GitLab repo will have the /ee prefix here, and pages that end with .html extensions include these in the metatag.

Example pages from each project:

Project	Example pages
GitLab	http://localhost:1313/administration/reference_architectures, http://localhost:1313/install/cloud_providers/
charts	http://localhost:1313/charts/, http://localhost:1313/charts/installation/upgrade
omnibus	http://localhost:1313/omnibus/, http://localhost:1313/omnibus/settings/
operator	http://localhost:1313/operator/, http://localhost:1313/operator/gitlab_upgrades/
gitlab-runner	http://localhost:1313/runner/configuration/feature-flags/, http://localhost:1313/runner/commands/

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.