Add platform/Span documentation.

Author: pan-

docs/reference/api/platform/Span.md

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+## Span
+
+A Span is a nonowning view to a sequence of contiguous elements.
+
+It can replace the traditional pair of pointer and size arguments passed as
+array definitions in function calls.
+
+### Construction
+
+Span objects can be constructed from a reference to a C++ array, a pointer to the
+sequence viewed and its size or the range of the sequence viewed:
+
+```
+const uint8_t str[] = "Hello mbed!";
+
+Span<const uint8_t> span_from_array(str);
+Span<const uint8_t> span_from_ptr_and_size(str, sizeof(str));
+Span<const uint8_t> span_from_range(str, str + sizeof(str));
+```
+
+### Operations
+
+Span objects can be copied and assigned like regular value types with the help
+of the copy constructor or the copy assignment (=) operator.
+
+```
+const uint8_t str[] = "Hello mbed!";
+
+Span<uint8_t> str_span(hello_mbed);
+Span<uint8_t> copy_constructed_span(str_span);
+Span<uint8_t> copy_assigned_span;
+
+copy_assigned_span = str_span;
+```
+
+You can retrieve elements of the object with the subscript ([]) operator. You
+can access the pointer to the first element of the sequence viewed with `data()`.
+The function `size()` returns the number of elements in the sequence, and
+`empty()` informs whether there is any element in the sequence.
+
+```
+void process_unit(uint8_t);
+
+void process(const Span<uint8_t> &data)
+{
+    if (data.empty()) {
+        // nothing to process
+        return;
+    }
+
+    for (ptrdiff_t i = 0; i < data.size(); ++i) {
+        process_unit(data[i]);
+    }
+}
+```
+
+You can slice Span from the beginning of the sequence (`first()`), from the end
+of the sequence (`last()`) or from an arbitrary point in the sequence (`subspan()`).
+
+```
+const uint8_t str[] = "Hello mbed!";
+
+Span<uint8_t> str_span(hello_mbed);
+
+ptrdiff_t half_size = str_span.size() / 2;
+
+Span<uint8_t> first_half = str_span.first(half_size);
+Span<uint8_t> second_half = str_span.last(half_size);
+Span<uint8_t> middle_half = str_span.subspan(/* offset */ half_size / 2, half_size);
+```
+
+### Size encoding
+
+The size of the sequence can be encoded in the type itself or in the value of
+the instance with the help of the template parameter Extent:
+  - `Span<uint8_t, 6>`: Span over a sequence of 6 `uint8_t`.
+  - `Span<uint8_t>`: Span over an arbitrary long sequence of `uint8_t`.
+
+When the size is encoded in the type itself, it is guaranteed that the Span
+view is a valid sequence (not empty() and not NULL) - unless Extent equals 0.
+The type system also prevents automatic conversion from Span of different
+sizes. Finally, the Span object is internally represented as a single pointer.
+
+```
+Span<uint8_t> long_span;
+
+// illegal
+Span<uint8_t, 6> span_mac_address;
+Span<uint8_t, 6> from_long_span(long_span);
+
+// legal
+uint8_t mac_address[6] = { };
+Span<uint8_t, 6> span_mac_address(mac_address);
+long_span = span_mac_address;
+```
+
+When the size of the sequence viewed is encoded in the Span value, Span
+instances can view an empty sequence. The function empty() helps client code
+decide whether Span is viewing valid content or not.
+
+### Span class reference
+
+[![View code](https://www.mbed.com/embed/?type=library)](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/class_span.html)
+