docs: cross-reference json_view and json_document for 3.13.0 (#5892)

* docs: cross-reference json_view and json_document for 3.13.0

Document the new zero-copy document/view API across the docs: add a
"Documents and views" section to the basic_json API hub linking to all
specializations, point parsing/conversion/modifying pages to the view-based
alternatives, add document/view sections to feature pages, update the
landing page to mention json_view.hpp/json_literals.hpp, and cross-link
debugging/exceptions.

Also document wide-string inputs (and ill-formed UTF-16/32 rejection),
add parse/accept examples for wide strings, fix related doc gaps, and
align features/parsing and FAQ with the added support.

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

* docs: fix article before std::istream in FAQ

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

* docs: clarify document inputs, decoding, get<T>() and editing

Address review comments: say which inputs are borrowed and that a wide
string pointer is an extra input, which values are decoded during parsing,
that get<T>() reads only built-in types directly, all forms of set() and
that the document's insert() is array-only, and how to store wide strings.

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

* docs: integers are converted while parsing a document

Also say "They accept every input" in basic_json_document::parse.

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

---------

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

docs/mkdocs/docs/examples/accept__wide_string.cpp

@@ -0,0 +1,18 @@
+#include <iostream>
+#include <string>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // a valid JSON text, given as UTF-16
+    const std::u16string valid_text = u"{\"name\":\"\u00e9\"}";
+
+    // an invalid JSON text, given as UTF-16
+    const std::u16string invalid_text = u"{\"name\":\"\u00e9\",}";
+
+    std::cout << std::boolalpha
+              << json::accept(valid_text) << ' '
+              << json::accept(invalid_text) << '\n';
+}

docs/mkdocs/docs/examples/accept__wide_string.output

@@ -0,0 +1 @@
+true false

docs/mkdocs/docs/examples/parse__wide_string.cpp

@@ -0,0 +1,27 @@
+#include <iostream>
+#include <string>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // the same JSON text, given in different encodings
+    const std::string utf8 = "{\"name\":\"\u00e9\"}";
+    const std::u16string utf16 = u"{\"name\":\"\u00e9\"}";
+    const std::u32string utf32 = U"{\"name\":\"\u00e9\"}";
+    const std::wstring wide = L"{\"name\":\"\u00e9\"}";
+
+    // the encoding is deduced from the character type of the input, and all
+    // of them yield the same value
+    std::cout << json::parse(utf8) << '\n'
+              << json::parse(utf16) << '\n'
+              << json::parse(utf32) << '\n'
+              << json::parse(wide) << '\n';
+
+    // a wide string literal works as well: u"..." is UTF-16, U"..." is UTF-32
+    std::cout << json::parse(u"[1, 2, 3]") << '\n';
+
+    // iterators over wide characters work as well
+    std::cout << json::parse(utf16.begin(), utf16.end()) << '\n';
+}

docs/mkdocs/docs/examples/parse__wide_string.output

@@ -0,0 +1,6 @@
+{"name":"é"}
+{"name":"é"}
+{"name":"é"}
+{"name":"é"}
+[1,2,3]
+{"name":"é"}