Add a complete list of acknowledgements for financial and other support

[rgommers]

Includes:

• CI providers (very significant support in terms of compute resources)
• Google for Season of Docs
• IBM for SIMD bounty
• perhaps some of the entities on https://scipy.org/scipylib/donations.html#acknowledgements

Related: I have to complete the sponsorship policy, which should spell out the criteria for being listed under Sponsors. The list above will be less visible, probably reachable from a text link under the sponsor logos on https://numpy.org/about/