Add the encode-intent-in-vis coding guideline

[tatetian]

This PR adds a new coding guideline, encode-intent-in-vis, to the maintainability guidelines: a visibility modifier in Asterinas declares an item's maximum intended exposure, regardless of what its enclosing modules happen to allow. It grew out of the discussion in #3710.

This PR includes a secondary change that reorders the guidelines in the "Crates & Modules" pages to make related guidelines close to each other.

[Copilot]

Pull request overview

This PR extends Asterinas’s maintainability coding guidelines by introducing a new Rust guideline, encode-intent-in-vis, clarifying that visibility modifiers should express an item’s maximum intended exposure (not merely its current effective exposure given module nesting).

Changes:

• Add the encode-intent-in-vis guideline to the “Crates & Modules” Rust-specific maintainability guidance, including rationale and an exception for struct/union fields.
• Add the new guideline to the maintainability guideline index so it appears in the quick “one-line gist” list.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
book/src/to-contribute/coding-guidelines/for-maintainability/rust-specific/crates-and-modules.md	Adds the new visibility-intent guideline text and supporting explanations/links.
book/src/to-contribute/coding-guidelines/for-maintainability/README.md	Adds encode-intent-in-vis to the maintainability guideline index list.

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[tatetian]

We reached a consensus with this new guideline in #3710. So I am merging this new guideline directly.