Name the source and license of adapted code in the file comments (#5965)

The files with code adapted from fast_float, Zmij and simdjson carried the
authors' SPDX copyright lines, but their file comments did not say what
was taken or under which license: number_parse.hpp, to_chars.hpp,
simd_scan.hpp and view/simd.hpp now do, as zmij.hpp and the yyjson-based
view files already did. README and the license page now also name
fast_float's eight-digit parsing and the lexer's use of simdjson's UTF-8
check. Comments only.

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

include/nlohmann/detail/input/simd_scan.hpp

@@ -30,6 +30,13 @@
 // NLOHMANN_JSON_NO_TARGET_ATTRIBUTE (for compilers that ignore the target
 // attribute and then reject the SSSE3 intrinsics) keeps the check portable
 // without disabling the SSE2 code.
+//
+// The vector UTF-8 check (utf8_lookup4) is adapted from simdjson
+// (https://github.com/simdjson/simdjson, Daniel Lemire, Geoff Langdale, John
+// Keiser, and contributors, available under the Apache 2.0 and MIT licenses;
+// used here under the MIT license): the "lookup4" algorithm and tables of
+// J. Keiser and D. Lemire, "Validating UTF-8 In Less Than One Instruction Per
+// Byte" (2021).
 #if !defined(JSON_NO_SIMD) && defined(__aarch64__) && (defined(__GNUC__) || defined(__clang__)) && defined(__BYTE_ORDER__) && defined(__ORDER_LITTLE_ENDIAN__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
     #include <arm_neon.h>
     #define JSON_SCAN_NEON 1