fix: Proper z-indexing on 'Back to main menu' button in mobile

[garikAsplund]

Fixes #1231 by adding z-index of 2 to mobile nav back-button. Also fixes same issue that wasn't mentioned with Tutorial submenu headers covering same button.

Before:

After:

The h2 used a z-index of 1, so I just went with 2.

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[7nik]

Thank you!