Deslopify index page by using simpler prose from original docs

[Feel-ix-343]

Deslopify index page by using simpler prose from original docs

Summary

Replaced the accordion/card-heavy index page with simpler, more direct prose from the original documentation. The new version removes the fancy MDX components (AccordionGroup, Accordion, CardGroup, Card, Tabs) and uses straightforward markdown with Obsidian-style wikilink embeds and callouts instead.

Key changes:

• Removed "Why Markdown-Oxide?" accordion section and replaced with direct prose
• Replaced "Key Features" accordions with simpler section structure using wikilink embeds to Features Index
• Removed "Quick Start" tabs section with editor-specific instructions
• Removed "Community & Support" card group
• Changed from marketing-focused copy to more technical, documentation-focused prose

Review & Testing Checklist for Human

⚠️ HIGH PRIORITY - Documentation Rendering:

• Verify the page renders correctly on oxide.md - The new content uses Obsidian-style syntax (wikilink embeds like ![[Features Index#^linking]] and callouts like > [!note]) which may not be supported by the MDX documentation system
• Check all wikilink embeds display properly - There are ~10 wikilink embeds that reference the Features Index file with block references (e.g., ^linking, ^unresolvedCompletions, etc.)
• Verify callout blocks render - The page uses > [!note] and > [!info] callout syntax
• Test the "Setup Instructions.md" link - The link uses angle bracket syntax: [here](<Setup Instructions.md>)
• Confirm no critical content was lost - The Quick Start section with installation instructions and Community & Support links were removed

Test Plan

1. Deploy to staging/preview environment if available
2. Navigate to oxide.md/index and verify the page loads without errors
3. Check that all embedded content from Features Index appears correctly
4. Verify all links are clickable and navigate to the correct pages
5. Compare with the original "sloppy" version to ensure no essential information was lost

Notes

• This PR was created based on the request to "deslopify" the index page by using the exact prose from the current site
• The prose was taken from docs/Markdown Oxide Docs/index.md which uses Obsidian formatting
• Major risk: The Obsidian-style syntax may not be compatible with the MDX documentation renderer, which could break the entire page
• Link to Devin run: https://app.devin.ai/sessions/a9674e165eed4ab2a98d9c17c6eec83a
• Requested by: Felix Zeller (felix@exa.ai) / @Feel-ix-343