docs: fix broken links and update outdated docker command

[alchemistlethal-a11y]

Documentation Audit & Remediation

This PR addresses several minor issues identified during a documentation audit:

1. Fixed Broken Links in docs/intro.md:

   • Added missing .md extensions to internal links to ensure correct navigation when viewing files on GitHub.
   • Corrected relative paths for several key features and getting-started guides.

2. Fixed Broken Links in docs/getting-started/browser-extension-installation.md:

   • Corrected absolute-style paths (/key-features/...) to relative paths (../key-features/...) to ensure they resolve correctly within the documentation structure.
   • Fixed the PWA link to include the .md extension.

3. Updated Outdated Docker Command in package.json:

   • Updated the docker:run script to use version 0.19.0, matching the project's current version and the latest release referenced in the README.

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