Use frontmatter menu information to generate the navbar menu instead of hardcoding #50228
Description
Right now, in the navbar, the menu list is generated from a hardcoded list. Docsy provides a better way in which the front matter of the section _index.md files contains a manu param which provides enough information to generate the main menu list dynamically.
We should rework navbar.html to adopt that style.
The main blocker is the section _index.md files having that information in the front matter as without that, that section won't show up as a main menu item. This seems to be no issue for the en site but we need to ensure that other localisations also have this. Alternatively , we need a workaround for those particular localisations.
Also see #50097 (comment)