docs: clarify git process environment defaults

[babblebey]

This pull request updates the documentation in README.md to clarify how Git process environment defaults are handled in CI workflows when using @semantic-release/core. The changes reorganize and expand the explanation of environment variable defaults, making it clearer for users how and when to set or override these values.

Key documentation improvements:

Clarification and reorganization of Git environment variable defaults:

• Moved and expanded the explanation of Git process environment defaults to a dedicated section, providing a clearer example of how callers should set these variables for CI environments.
• Updated the example email addresses in the environment variable setup to use the official semantic-release.org domain for clarity and consistency.
• Clarified that core applies its own defaults for any Git environment values not set by the caller, and that these remain overridable. [1] [2]

Streamlining and removal of redundant documentation:

• Removed redundant and outdated explanations regarding environment variable handling from earlier sections to avoid confusion and duplication. [1] [2]
• Updated references to environment variable hardening in the dependency and ownership model section to point to the new, dedicated section for easier navigation.

[github-actions]

🎉 This PR is included in version 1.0.0-beta.3 🎉

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

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