Docs: normalize internal links to extensionless paths and update PlatformIO install link

[LucHeart]

Motivation

• Standardize internal documentation links to extensionless paths to match the site's routing and avoid stale .md references.
• Fix an outdated PlatformIO IDE marketplace link to the official installer URL.
• Ensure consistency across hardware, guides, vendors and home pages so internal linking behaves predictably.

Description

• Replaced numerous internal *.md links with extensionless paths across docs in guides, hardware, vendors, home, docs/dev, and other directories to align with the site's routing conventions.
• Updated the PlatformIO IDE link in docs/dev/contributing/compile-firmware.mdx to https://platformio.org/install/ide?install=vscode.
• Normalized various relative links and table entries (e.g., boards index and vendor pages) to remove .md extensions and ensure consistent link formatting.
• Adjusted a few image and card targets to use the new extensionless targets where applicable.

Testing

• Built the documentation site locally with yarn build to validate rendering and routing, and the build completed successfully.
• Ran a link-checker (yarn run linkcheck) against the generated site to detect broken internal links, and no broken internal links were reported.

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Codex Task

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