Rename Error::provide to Error::provide_context

[c4rtune]

Resolves the deref/autoref method name conflict between Error::provide and Provider::provide mentioned in the tracking issue for error_generic_member_access rust-lang/rust#99301.

Note: provide_context is the method name that was originally proposed in the RFC.

This comment suggests removing the provider API, which would also resolve the name conflict as a side effect. However, even if it is indeed removed, there's still a small chance it'll be brought back later, I guess. Plus, some users (including myself) feel like the name provide_context provides a clearer indication what should be provided.