[Docs] reorganize sponsorship page

[simon-mo]

This is not yet the desired final state but we can iterate on it a bit later. I want to add some levels of "tiers" and "referral" to each sponsor.

This PR adds Novita as sponsor and highlight Anyscale for Slack Sponsorship.

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
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[WoosukKwon]

LGTM!