Copy icons.svg to expected location

What does this MR do and why?

Fixes a regression introduced in !76 (merged). We still need icons.svg to exist at the site root.

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 clean to wipe all compiled assets that will be recreated at build time.
3. Run make view and verify icons load (e.g, chevrons on the menu).
4. Run make view-prod and verify icons load when the site is running in a subdirectory.

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.