Follow-up: Document local artifact reference for gitlab-vscode-extension build"

Problem to solve

The Web IDE can reference local builds of the GitLab VSCode Extension for testing purposes but this developer tool is not documented.

Solution proposal

Write documentation to spread the knowledge of this tool.

References

The following discussion from !569 (merged) should be addressed:

• @ealcantara started a discussion: (+1 comment)

  @viktomas Could you maintainerize/merge this MR 🙏 ?