Return the first member of duplicate keys from lookups again

Looking up the last member of a duplicate key cannot stop at a match, so
every lookup scanned the whole object (1.6 to 3.4 times slower for small
objects). operator[](key), at, find, value, contains, count, and JSON
pointer resolution return the first member again, as yyjson and simdjson
do; materialize() and get<map>() keep the last value, as parse().

The documentation says so in the feature page and on each lookup page,
and explains how to get the value parse() would give. The integer index
templates and the discarded chaining of operator[] stay.

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

docs/mkdocs/docs/examples/basic_json_view__items.cpp

@@ -7,8 +7,8 @@ int main()
 {
     // a settings object whose source text records every update to a key as
     // a duplicate member. items() visits all of them, in document order, so
-    // the update history is visible; operator[] and materialize() -- like
-    // basic_json::parse() -- see the last one
+    // the update history is visible; operator[] only ever sees the first
+    // one, and materialize() -- like basic_json::parse() -- keeps the last
     json_document updates = json_document::parse(R"({"retries": 1, "timeout": 30, "retries": 5})");
     const auto settings = updates.root();
 
@@ -17,6 +17,6 @@ int main()
         std::cout << item.key() << '=' << item.value().materialize().dump() << '\n';
     }
 
-    std::cout << "last \"retries\" seen by operator[]: " << settings["retries"].materialize().dump() << '\n';
+    std::cout << "first \"retries\" seen by operator[]: " << settings["retries"].materialize().dump() << '\n';
     std::cout << "last \"retries\" kept by materialize(): " << settings.materialize()["retries"].dump() << '\n';
 }