improve documentation of slices

docs/reference/src/code/language/built-ins/slices/.gitignore

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+out
+target

docs/reference/src/code/language/built-ins/slices/Forc.lock

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+[[package]]
+name = 'arrays'
+source = 'root'
+dependencies = ['std']
+
+[[package]]
+name = 'core'
+source = 'path+from-root-54F92B42A645EA2B'
+dependencies = []
+
+[[package]]
+name = 'std'
+source = 'git+https://github.com/fuellabs/sway?tag=v0.24.5#e695606d8884a18664f6231681333a784e623bc9'
+dependencies = ['core']

docs/reference/src/code/language/built-ins/slices/Forc.toml

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+[project]
+authors = ["Fuel Labs <contact@fuel.sh>"]
+entry = "lib.sw"
+license = "Apache-2.0"
+name = "slices"
+
+[dependencies]
+std = { path = "../../../../../../../sway-lib-std" }

docs/reference/src/code/language/built-ins/slices/src/lib.sw

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+library;
+
+// ANCHOR: syntax
+fn syntax(s: &[u64]) -> u64 {
+    s.len()
+}
+// ANCHOR_END: syntax

docs/reference/src/documentation/language/built-ins/slices.md

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+# Slices
+
+A slice is similar to an [array](arrays.md), in the sense that it is a contiguous sequence of elements of the same type.
+
+Unlike arrays, slices cannot be allocated, because its size is unknown at compilation time. The only way to use slices is through references to a slice.
+
+References to slice are "fat pointers" containing two items:
+
+- a pointer to the first element of the slice;
+- a `u64` with how many elements the slice has.
+
+```sway
+{{#include ../../../code/language/built-ins/slices/src/lib.sw:syntax}}
+```