docs: add dark mode version of BrowserStack logo

[fredrikstave]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

I saw something that bugged me in the README and I knew how to easily fix it 🙂

Dark logo on dark background 😢

Test Plan

• Open two browser windows side by side
• In one open your profile settings, in the other open this pull request
• Go to the README.md file and preview the changes
• Cycle through the different theme settings in the second tab, while refreshing the pull request tab
• You should see the BrowserStack logo at the bottom of the README change from light to dark
• (Bonus) If you want to try syncing appearance with the system settings as well, it should update instantly
• Check that the added code looks okay

Test links

Profile settings link: https://github.com/settings/appearance

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 89	🟢 97	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 82	🟢 100	🟢 100	🟢 100	🟢 90	Report

[slorber]

😄 thanks