[infra] fix: maintenance page could not load static assets

[amatissart]

Description

The maintenance mode was not defined properly in Nginx configuration: all assets would redirect return a response code 503, causing the browser to not display them.

This PR defines a separate location /maintenance section to serve the static page as expected.

Checklist

• I added the related issue(s) id in the related issues section (if any)
  • if not, delete the related issues section
• I described my changes and my decisions in the PR description
• I read the development guidelines of the CONTRIBUTING.md
• The tests pass and have been updated if relevant
• The code quality check pass