Fix typos and capitalization across documentation

[him2him2]

Summary

Fix various typos, grammar issues, and capitalization errors across 21 documentation files:

• "Github" → "GitHub" capitalization fix throughout (about.md, git.md, about-this-guide.md, debugging-support-in-rustc.md, and all notification-groups files)
• "its self" → "itself" (macro-expansion.md, what-bootstrapping-does.md)
• "MIRI" → "Miri" — correct project name capitalization (overview.md, what-bootstrapping-does.md)
• Grammar fixes: "to given" → "to give", "an user-specified" → "a user-specified", "an separate" → "a separate" (diagnostics.md, git.md)
• Incorrect attribute syntax: !#[warn → #![warn (diagnostics.md)
• "overrides" → "override" subject-verb agreement (suggested.md)
• "issue commit" → "commit message" (contributing.md)
• Double space removal (ty.md)

[rustbot]

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