Fix .bd-sizebar-secondary and .secondary-toggle breakpoint

[sinsong]

My friend find that .bd-sidebar-secondary collapse before .secondary-toggle appear. Which means, viewport width between 1200px (xl breakpoint) and 992px (lg breakpoint), both sidebar and toggle button disappear.

I find that a rule come from pydata-sphinx-theme interact with .bd-sidebar-secondary cause collapse less that 1200px (exactly at 1199.98px). With the breakpoint xl.

So this PR is for synchronize .secondary-toogle button with .bd-sidebar-secondary collapse through align breakpoint to xl.

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[agoose77]

This matches my read of the upstream code, which sets $breakpoint-sidebar-secondary: xl;: https://github.com/pydata/pydata-sphinx-theme/blob/05fd48a8159d6d4046a06df6688efae0dacf34eb/src/pydata_sphinx_theme/assets/styles/variables/_layout.scss#L21-L22