docs(zval): add FromZVal / FromZValMut guidance in _04_zval

[Copilot]

This updates the ZVal documentation to explicitly cover trait-based generic conversion from ZVal. The guide now documents when to use FromZVal/FromZValMut alongside expect_* helpers.

• What changed

  • Added a new subsection in phper-doc/doc/_04_zval/index.md:
    • Generic conversion via FromZVal / FromZValMut
  • Documented trait roles clearly:
    • FromZVal: conversion from &ZVal
    • FromZValMut: conversion from &mut ZVal
  • Documented the corresponding generic APIs:
    • ZVal::expect_type
    • ZVal::expect_mut_type

• Example added

  • Included a focused Rust snippet showing generic immutable/mutable conversion helpers and usage:

use phper::values::{FromZVal, FromZValMut, ZVal};

fn expect_immutable<'a, T: FromZVal<'a>>(val: &'a ZVal) -> phper::Result<T> {
    val.expect_type()
}

fn expect_mutable<'a, T: FromZValMut<'a>>(val: &'a mut ZVal) -> phper::Result<T> {
    val.expect_mut_type()
}

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