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Keep the desktop docs side menu below the sticky header

Fixes #1407

Description

I increased the desktop offset for the API docs side affix so its first entries no longer sit underneath the wrapped sticky header at viewport widths of 1200px and above.

I verified the change against the current 4.8.0-beta00018 docs page by replacing its stylesheet in headless Chromium with this branch's stylesheet. At 1200, 1215, 1280, 1350, and 1600px, the 195px-tall header ends before the side menu starts at 200px, and every visible menu entry remains inside the viewport.

Validation

• uvx prek run --files websites/apidocs/Templates/LuceneTemplateAssets/styles/main.css
• Headless Chromium layout regression check at 1200, 1215, 1280, 1350, and 1600px
• git diff --check origin/master...HEAD

I did not run the complete Windows-based DocFX website build locally; the browser regression check exercises the changed stylesheet against the published docs markup.