a11y: #quarto-margin-sidebar, #quarto-search-results, #quarto-sidebar-glass are outside every landmark (region)

[cderv]

Reported in #14373.

Repro: https://github.com/alexpghayes/quarto-accessibility-reprex

axe rule: region · impact: moderate · tag: best-practice
Targets:

• <div id="quarto-margin-sidebar" class="sidebar margin-sidebar"> — TOC, margin notes
• <div id="quarto-search-results"> — search results panel
• <div id="quarto-sidebar-glass"> — decorative overlay

These are plain <div> elements without landmark roles, so their contents sit outside every landmark. The margin sidebar and search results should adopt role="complementary" (or equivalent) with a descriptive aria-label. The sidebar-glass overlay is decorative and should be aria-hidden="true".

Part of #8706.