Update Project API documentation to support `sha256` object format
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The following discussion from !137659 should be addressed:
- [ ] @msedlakjakubowski started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/137659#note_1747483344):
**Task**
We should update the documentation to mention SHA256 support of Projects creation API.
This feature is currently released behind the feature flag `support_sha256_repositories`: https://gitlab.com/gitlab-org/gitlab/-/issues/431864.
It's blocked by https://gitlab.com/gitlab-org/gitlab/-/issues/431864+.
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