Fix accessibility in header improvement

[Stef-Rousset]

🎩 What? Why?

This PR improves accessibility in header page by adding role attribute on div.main-bar__search, and by changing div.main-bar__links-desktop for nav.main-bar__links-desktop with role="navigation"

This PR comes from the audit of Angers city (page 22) and refers on criteria 1.3.1 from WCAG

📌 Related Issues

• Related to https://github.com/orgs/OpenSourcePolitics/projects/26/views/1?pane=issue&itemId=122480113&issue=OpenSourcePolitics%7Cintern-tasks%7C82

Testing

1. As a user, go to home page and open dev tools
2. Check in the header that the search div has a role="search" and that the links are in a nav element with role="navigation"

📷 Screenshots (optional)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions. @carolromero & @xabier feel free to chime in.