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__is_discarded.cpp

@@ -0,0 +1,19 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+
+int main()
+{
+    std::cout << std::boolalpha;
+
+    // with allow_exceptions == false, a parse error produces a discarded
+    // document instead of throwing -- exactly like basic_json::parse()
+    json_document doc = json_document::parse(R"({"a": )", /* allow_exceptions */ false);
+    std::cout << doc.is_discarded() << '\n';
+    std::cout << doc.root().is_discarded() << '\n';
+
+    // a successful parse is never discarded
+    json_document ok = json_document::parse(R"({"a": 1})", false);
+    std::cout << ok.is_discarded() << '\n';
+}