[openvsx] Website: More aggressive push for sponsorship

Open VSX is used by millions of users worldwide. There is an specific sponsorship linked from the Open VSX homepage:

https://www.eclipse.org/donate/openvsx/

I think we should find ways to push sponsorships more aggressively to help sustain the service.

Some ideas: after X visits to a page, forcibly show the donation page; put a prominent notice before search results, place a notice on very popular extensions, and other such industry-accepted practices.

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/cc @thomasfroment