added text-shadow to docs mobile navigation text

[deployn]

[DOCS]/[MOBILE-NAVIGATION]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Only
• Refactor

Description, Motivation and Context

Added text-shadow property to the mobile navigation in the documentation.
In light-mode, the mobile navigation text is hard to read.
Accessibility is still not optimal, but a little better.

Screenshots

Before:

After:

[jrgarciadev]

Hey @deployn I appreciate your contribution, however, this contrast issue will be fixed in the next release thanks to the new theme palette #443