Improve path handling for navigation

What does this MR do and why?

Ensures navigation links work when the site is deployed in a subdirectory (like docs.gitlab.com/15.0, or in the short term, https://gitlab-org.gitlab.io/technical-writing-group/gitlab-docs-hugo/).

• Adds a metatag to store the base URL (generated by Hugo), and uses this to make navigation links absolute
• Adjusts Vite configuration to use dynamic base path
• Adds shortcut for running a subdirectory-style site locally (make view-prod)

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. Run make view and verify the site and navigation works at on the bare domain: http://localhost:1313
3. Run make view-prod and verify the site and navigation works in a subdirectory: http://localhost:1313/technical-writing-group/gitlab-docs-hugo/

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.