feat: add some utility functions for docs

[QuadnucYard]

Added

• Doc::is_nil
• DocBuilder::into_ref to make inner ref, which can help prepare for cloning.
• [T; N]::pretty, making it easier to concat.

[gemini-code-assist]

Summary of Changes

Hello @QuadnucYard, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces several new utility functions aimed at enhancing the usability and flexibility of document manipulation within the library. The changes provide more ergonomic ways to inspect document states, prepare documents for advanced operations, and construct documents from common data structures like arrays.

Highlights

• New Doc utility: I've added an is_nil method to the Doc type, which provides a convenient way to check if a document is the Nil variant. This can simplify conditional logic when working with documents.
• New DocBuilder utility: I've introduced an into_ref method for DocBuilder. This function allows converting a DocBuilder into a new DocBuilder that holds an internal reference to the original document, which can be useful for preparing documents for cloning or other operations requiring a reference.
• Array Pretty implementation: I've implemented the Pretty trait for fixed-size arrays ([T; N]). This enables arrays of Pretty-implementing types to be directly converted into a DocBuilder by concatenating their elements, simplifying the creation of documents from array data.

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[gemini-code-assist]

Code Review

This pull request introduces utility functions for document handling, including checking for nil documents and creating references. The changes are well-implemented, with suggestions provided for efficiency and test coverage.