[UI-Shell] update navigation items

[techtolentino]

Feature description

• related to this issue in the Qiskit.org repo
  • specifically this comment
• also related to Mega dropdown menu navigation #160

Main menu updates:

• The link to the Providers page will be near the Documentation link, on the right side
• The links to Providers and Core will be removed from the Documentation drop-down menu. Not yet

Additional context

Here we need to decide if the block will continue having the size of the screen or if we want that block to adjust to the content on the submenu. Update: We don't need to remove the providers and core section from the menu, so the size will remain the same

Note: possibly related is this issue from @HuangJunye, regarding the width and usability of the UI Shell Documentation dropdown

[HuangJunye]

Thanks for the ping @techtolentino. Did you want to link #160 to the description?