Document element access, lookup, and iteration of json_view

- API pages for operator[], at, front, back, find, contains, count,
  begin, end, cbegin, cend, items, and type_name of basic_json_view,
  linked both ways with the basic_json pages
- the feature page and size() describe document order and duplicate
  keys
- the examples show when the view helps: reading a few fields of a large
  text, probing optional members, and members in source order

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_view/cbegin.md

@@ -0,0 +1,49 @@
+# <small>nlohmann::basic_json_view::</small>cbegin
+
+```cpp
+iterator cbegin() const noexcept;
+```
+
+Returns an iterator to the first element, in [document order](begin.md). Equivalent to [`begin()`](begin.md): a view
+is always read-only, so `#!cpp const_iterator` and `#!cpp iterator` are the same type, and `cbegin()` exists only so
+that generic code that expects a `cbegin()`/`cend()` pair works with a view too.
+
+## Return value
+
+Iterator to the first element; identical to what [`begin()`](begin.md) returns.
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Examples
+
+??? example
+
+    The example below sums many measurements with `std::accumulate`, using `cbegin()`/[`cend()`](cend.md) as the
+    range -- the same way it would for any standard container -- without ever materializing the whole array into a
+    `BasicJsonType` value.
+
+    ```cpp
+    --8<-- "examples/basic_json_view__cbegin.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/basic_json_view__cbegin.output"
+    ```
+
+## See also
+
+- [begin](begin.md) - returns an iterator to the first element
+- [cend](cend.md) - returns a const iterator to one past the last element
+- [`BasicJsonType::cbegin`](../basic_json/cbegin.md) - the corresponding function of `basic_json`
+
+## Version history
+
+- Added in version 3.13.0.