Fix homepage hero/sidebar overlap in desktop layout

[ttuff]

Motivation

• The homepage hero was placed outside the two-column wrapper and visually extended underneath the left "Tags" sidebar at desktop widths.
• Existing full-bleed hero rules (100vw / negative margins / pseudo-element) allowed the hero background to span beneath the sidebar instead of being constrained to the main content column.

Description

• Moved the hero markup into the right column by wrapping the hero inside the .oasis-layout > .oasis-main container in docs/index.md so the hero sits within the main content column.
• Added a single sidebar width variable --oasis-sidebar-w and authoritative desktop two-column flex layout in docs/styles/extra.css, defining display:flex on .oasis-layout and sizing for .oasis-sidebar and .oasis-main.
• Scoped the header-flush hero offset to .oasis-main .oasis-hero and removed the earlier width:100vw and negative margins, replacing them with width:100% and zeroed margins so the hero no longer forces a full-bleed layout.
• Constrained any pseudo-element full-bleed behavior by adding .oasis-main .oasis-hero::before rules at desktop sizes so the hero background/pseudo-element starts at the main column left edge and does not extend under the sidebar.

Testing

• Ran mkdocs build successfully to ensure the site builds with the CSS and markup changes.
• Launched a dev server (mkdocs serve) and captured a desktop screenshot via Playwright to visually confirm the hero background and text no longer sit beneath the left tags sidebar.
• Verified diffs for docs/index.md and docs/styles/extra.css to confirm the expected markup moves and CSS updates were applied.

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