Improve pagination nav with disabled state and accesskeys

[julioest]

Changes:

• Add disabled state styling for "Up" button even if no parent page exists
• Fix accesskey attributes (p/u/n instead of all "p")
• Improve template structure with proper conditional logic
• Add accessibility attributes for disabled state

Issue

• References make space for up arrow in docs website-v2#1837

Preview:

Site docs:

Library doc:

Ref boostorg:

[cppalliance-bot]

An automated preview of the documentation is available at https://469.site-docs.prtest2.cppalliance.org/index.html

[karimarie67]

The arrows look good and are functional.