Adding linking for Once docs #29377

[projektir]

Linking everything around Once, ONCE_INIT, and OnceState.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @BurntSushi (or someone else) soon.

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[bors]

📌 Commit 3207657 has been approved by estebank

[bors]

💡 This pull request was already approved, no need to approve it again.

• There's another pull request that is currently being tested, blocking this pull request: change the strategy for diverging types #40224

[bors]

📌 Commit 3207657 has been approved by estebank

[bors]

💡 This pull request was already approved, no need to approve it again.

• There's another pull request that is currently being tested, blocking this pull request: change the strategy for diverging types #40224

[bors]

📌 Commit 3207657 has been approved by estebank

[estebank]

@bors r+ rollup

[bors]

📌 Commit 3207657 has been approved by estebank

[bors]

📌 Commit 3207657 has been approved by estebank