Closes 3474778 - Address header / megamenu mobile design

Start the work to include user menu, add user icons/search icon as placeholder, more mobile tweaking.

This further tweaks the main nav and mobile styling to be even more aligned with Figma for the main navigation and header.

TBM supports a link caption; that is what is shown in the screenshot. It can be put on any link.

Also has minor config updates here https://gitlab.com/drupal-infrastructure/sites/drupalorg/-/merge_requests/35 for the drupalorg repo.

Closes issue #3474778

Testing steps

1. Checkout the drupalorg version / branch.
2. Checkout this bluecheese MR's branch
3. composer recreate
4. Login as admin and sync all menu items
5. View the homepage menu and ensure it is more aligned with Figma