docs: remove references to nonexistent cat() buffer-composition function

Four pages cited a `cat()` function that was never part of the public
buffer API. The Antora build did not flag these because the references
appeared inside `[source,cpp]` blocks (not xrefs), but readers following
the examples would hit a compilation error.

Fixes:

* `5a.overview.adoc` — removed the `Zero-Allocation Composition`
  section; the conceptual point ("any composition of the above without
  allocation") is preserved in the bullet list above and in
  `5c.sequences.adoc`.
* `5c.sequences.adoc` — removed the `Zero-Allocation Composition`
  section; `Heterogeneous Composition` already covers the same idea.
* `5e.algorithms.adoc` — rewrote the Zero-Copy I/O example to use
  `std::array` directly, mirroring the Scatter/Gather example on the
  same page.
* `8c.buffer-composition.adoc` — removed the exercise that referenced
  `cat()`; the other two exercises remain.

Author: sgerbino

doc/modules/ROOT/pages/5.buffers/5a.overview.adoc

@@ -99,23 +99,6 @@ This single signature accepts:
 * A custom composite type
 * *Any composition of the above—without allocation*
 
-== Zero-Allocation Composition
-
-With concepts, composition creates views, not copies:
-
-[source,cpp]
-----
-HeaderBuffers headers = /* ... */;
-BodyBuffers body = /* ... */;
-
-// cat() creates a view that iterates both sequences
-auto combined = cat(headers, body);  // No allocation!
-
-write_data(combined);  // Works because combined satisfies ConstBufferSequence
-----
-
-The `cat` function returns a lightweight object that, when iterated, first yields buffers from `headers`, then from `body`. The buffers themselves are not copied—only iterators are composed.
-
 == STL Parallel
 
 This design follows Stepanov's insight from the STL: algorithms parameterized on concepts (iterators), not concrete types (containers), enable composition that concrete types forbid.

doc/modules/ROOT/pages/5.buffers/5c.sequences.adoc

@@ -133,23 +133,6 @@ task<std::size_t> read_all(Stream& stream, Buffers buffers)
 * Iteration over unconsumed buffers
 * `buffer_size()` of remaining bytes
 
-== Zero-Allocation Composition
-
-The `cat()` function composes buffer sequences without allocation:
-
-[source,cpp]
-----
-auto headers = std::array{header_buf1, header_buf2};
-auto body = body_buffer;
-
-auto combined = cat(headers, body);  // No allocation
-
-// combined satisfies ConstBufferSequence
-// Iteration yields: header_buf1, header_buf2, body_buffer
-----
-
-The returned object stores references (or small copies for single buffers) and iterates through the composed sequence on demand.
-
 == Why Bidirectional?
 
 The concepts require bidirectional ranges (not just forward ranges) for two reasons:

doc/modules/ROOT/pages/5.buffers/5e.algorithms.adoc

@@ -202,7 +202,8 @@ std::string header = build_header();
 std::vector<char> body = load_body();
 
 // No copying—header and body are written directly
-co_await write(stream, cat(make_buffer(header), make_buffer(body)));
+std::array buffers = {make_buffer(header), make_buffer(body)};
+co_await write(stream, buffers);
 ----
 
 === Scatter/Gather Operations

doc/modules/ROOT/pages/8.examples/8c.buffer-composition.adoc

@@ -223,8 +223,7 @@ Prepared 2 buffers with 128 bytes total capacity
 == Exercises
 
 1. Create a function that takes any `ConstBufferSequence` and prints its contents
-2. Measure the difference between copying data into a single buffer vs. using `cat()`
-3. Implement a simple message framing protocol using buffer composition
+2. Implement a simple message framing protocol using buffer composition
 
 == Next Steps