Avoid duplicate prev/next access keys

[Herrtian]

When prev/next buttons are shown in both locations, both the top and footer links currently render the same accesskey values. This keeps the access keys on the footer links and omits them from the duplicate top links when the location is both.

Checks:

• git diff --check
• uv run --with pytest --with sphinx --with-editable . pytest tests/test_builders.py::test_prev_next_accesskeys_are_unique_when_buttons_location_is_both
• PYTHONUTF8=1 uv run --with pytest --with sphinx --with-editable . pytest tests/test_builders.py

Fixes #1057