Document json_document and json_view

- API pages for basic_json_document and basic_json_view, one per member,
  and for the four aliases, each with an example
- features/json_view.md: the problem the view solves, ownership and
  lifetime, what matches basic_json::parse() and what differs, and when
  to choose json, ordered_json, SAX, or the view
- the examples show why one would use the view, not only how: borrowed
  vs. owned input, reading a few fields and materializing one subtree,
  reusing a document across many messages
- registered in the mkdocs navigation, llms.txt, the docset, the
  exceptions page (out_of_range.416), architecture.md, the integration
  page, and the README; the yyjson credit is added to the README and
  license.md

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

docs/mkdocs/docs/examples/ordered_json_document.cpp

@@ -0,0 +1,13 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using ordered_json_document = nlohmann::ordered_json_document;
+
+int main()
+{
+    // ordered_json_document is basic_json_document<nlohmann::ordered_json>:
+    // materialize() preserves the insertion (source) order of object keys,
+    // instead of sorting them like json_document does
+    ordered_json_document doc = ordered_json_document::parse(R"({"z": 1, "a": 2, "m": 3})");
+    std::cout << doc.root().materialize().dump() << '\n';
+}