Reorganizing information from the docs.

[hayleycd]

Summary

Addresses #326. Moving website information from the contributing page in the documents to the project readme.

Release Note

none.

Documentation

A related PR is being opened in the docs repo.

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[haydentherapper]

https://github.com/sigstore/sigstore-website/pull/328 for the CI fix

[haydentherapper]

Good to rebase.