Fix all remaining design and coding accessibility issues

[thibaudcolas]

Follow-up to wagtail#12157. We want the bakerydemo website to be free of design and coding related accessibility issues. Content issues are desirable so we can test the accessibility checker.

• [x] Fix color contrast / link not distinguishable in text issues showing up on recipe pages
• [x] Fix main navigation menu issues when a top-level menu item has sub pages configured to show in the menu (create a page as a child of "Contact us"). Or remove support for dropdown menus
• [x] (test all other pages of the site and report possible improvements here / fix the issues)

[albinazs]

Fixed issues:

• [x] Fix color contrast and link not distinguishable in text issues showing up on Recipe pages
• [x] Remove support for dropdown menus within the main navigation menu
• [x] Fix the aria-allowed attributes issue on the Contact us form page
• [x] Fix the missing table headers issue on the Breads index page
• [x] Fix the missing heading level 1 issue on the Recipes index page
• [x] Add missing headings to the Location pages
• [x] Update Twitter to X on the FE only (icon, label, field verbose name for admin). Var name deliberately kept as is for better readability

Other possible improvements:

• [ ] Add a skip main navigation link
• [ ] Fix not visible focus appearance for the .picture-card (used for blog items on the home page, location items on the location index page)
• [ ] BE changes related to the removal of the dropdown support: removing the show_in_menus field for the children pages

[albinazs]

Additional fixed issue:

[x] Add a skip main navigation link