feat: upgrade docsy and hugo + add theme toggle

[milinddethe15]

upgrades the versions of docsy to v0.10.0 and hugo to v0.125.4

• fixes Upgrade hugo and docsy version (add dark theme) #3954
• fixes Implement Dark/Light Mode Compatibility for Website #3692

Based on suggestions received in #3968.

Preview link: https://deploy-preview-3973--competent-brattain-de2d6d.netlify.app/
This upgrade added some issues in layouts. If the team/community wants to go ahead with this upgrade, I can try fixing those issues.

[google-oss-prow]

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[google-oss-prow]

Hi @milinddethe15. Thanks for your PR.

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[Arhell]

/ok-to-test