Add editable json_documents: set, push_back, insert, and erase

basic_json_document gets a second template parameter, Editable
(false by default), plus the aliases json_editable_document,
json_editable_view, ordered_json_editable_document and
ordered_json_editable_view.

Editable documents can change values and structure without
rewriting the source text: set()/push_back() on values, keys,
array indices and JSON pointers; insert() before an array
element; erase() of an object key, array index or JSON pointer.

New values and element sequences go into edit storage that the
document owns and never moves, so views keep referring to their
value across edits and a parsed node never moves. Read-only
documents walk the plain node array and are unaffected.

Strings are checked for UTF-8 on entry, so dump() of an editable
document never throws type_error.316. Binary values cannot be
stored (type_error.319).

A seeded differential test applies random edits to an editable
document and to the equivalent ordered_json and compares both
after every step.

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

docs/mkdocs/docs/api/ordered_json_editable_view.md

@@ -0,0 +1,40 @@
+# <small>nlohmann::</small>ordered_json_editable_view
+
+<small>Defined in header `<nlohmann/json_view.hpp>`</small>
+
+```cpp
+using ordered_json_editable_view = basic_json_view<ordered_json, true>;
+```
+
+This type is a [`basic_json_view`](basic_json_view/index.md) of a value of an
+[`ordered_json_editable_document`](ordered_json_editable_document.md), the corresponding view for
+[`ordered_json_view`](ordered_json_view.md) the way [`json_editable_view`](json_editable_view.md) is for
+[`json_view`](json_view.md).
+
+## Examples
+
+??? example
+
+    The example below is the same as [`ordered_json_editable_document`'s](ordered_json_editable_document.md): the
+    views `set` returns see the document's member order preserved on `materialize()`, unlike for a
+    [`json_editable_document`](json_editable_document.md).
+
+    ```cpp
+    --8<-- "examples/ordered_json_editable_document.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/ordered_json_editable_document.output"
+    ```
+
+## See also
+
+- [ordered_json_editable_document](ordered_json_editable_document.md) - the document type this view refers into
+- [ordered_json_view](ordered_json_view.md) - the corresponding read-only view
+- [json_editable_view](json_editable_view.md) - the corresponding view for `json_editable_document`
+
+## Version history
+
+Since version 3.13.0.