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/basic_json_document__owns_source.cpp

@@ -0,0 +1,24 @@
+#include <iostream>
+#include <string>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+
+int main()
+{
+    std::cout << std::boolalpha;
+
+    std::string text = R"({"a": 1})";
+
+    // borrowed: the document only points into `text`; `text` must outlive it
+    json_document borrowed = json_document::parse(text);
+    std::cout << borrowed.owns_source() << '\n';
+
+    // owned: parse_copy() always takes its own copy
+    json_document copied = json_document::parse_copy(text);
+    std::cout << copied.owns_source() << '\n';
+
+    // owned: an rvalue std::string is moved in, not copied, but still owned
+    json_document moved_in = json_document::parse(std::string(text));
+    std::cout << moved_in.owns_source() << '\n';
+}