Update "Feature guide" documentation

[henriquemoody]

The documentation was quite outdated after the many changes I made in the main branch. This commit will make sure it's up-to-date.

I'm also creating a "Placeholder pipes" documentation, so there's not too much content in the "Feature guide"

[Copilot]

Pull request overview

This PR updates the Feature Guide documentation to align with recent changes in the main branch and creates a new dedicated Placeholder Pipes documentation page to better organize content.

Key changes:

• Adds comprehensive documentation for placeholder pipes with detailed examples of all available modifiers
• Restructures and expands the Feature Guide with new sections on validation methods, smart validation, PHP attributes, result composition, and advanced features
• Improves code examples and adds clearer explanations of validator behavior and immutability

Reviewed changes

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

File	Description
docs/placeholder-pipes.md	New documentation file dedicated to explaining placeholder pipe modifiers with usage examples for raw, quote, trans, listOr, listAnd, and custom modifiers
docs/feature-guide.md	Major restructuring and expansion adding sections on validation methods, PHP attributes, result composition, prefixed shortcuts, custom templates for nested structures, naming validators, reusing validators, and advanced features like nested validation paths and helpful stack traces

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[alganet]

Some suggestions on terminology. Nothing critical.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

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[alganet]

Small nitpick on the emdash. You can ignore it and merge if you want.