Fix clang-tidy findings in documentation examples and replace redundant aliases

Apply clang-tidy fixes to the documentation examples:
- performance-avoid-endl: std::endl -> '\n'
- readability-qualified-auto: auto -> auto* / const auto*
- performance-for-range-copy: copy -> const reference
- google-readability-casting: C-style cast -> static_cast

Replace redundant aliases (readability-redundant-qualified-alias) such as
"using json = nlohmann::json;" with "using nlohmann::json;" in the
documentation, README, examples, tests, and benchmarks. Aliases at class
scope and aliases of other types (with_base_class_t etc.) are kept.

No changes to the library or its public API. All changed examples were
compiled and run; their output matches the stored .output files (except
max_size and meta, which depend on the platform and compiler).

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

tests/benchmarks/src/benchmarks.cpp

@@ -21,7 +21,7 @@
 #include <vector>
 #include <test_data.hpp>
 
-using json = nlohmann::json;
+using nlohmann::json;
 
 //////////////////////////////////////////////////////////////////////////////
 // parse JSON from file

tests/benchmarks/src/benchmarks_view.cpp

@@ -17,8 +17,8 @@
 #include <string>
 #include <test_data.hpp>
 
-using json = nlohmann::json;
-using json_document = nlohmann::json_document;
+using nlohmann::json;
+using nlohmann::json_document;
 
 static std::string read_file(const char* filename)
 {