Fix MSVC errors in image test and add image_check doc anchor

Move the raw string literal out of the CHECK macro (MSVC preprocessor),
cast 64-bit header fields to std::size_t (C4244 on 32-bit), and give the
image_check section in load.md a real anchor for the mkdocs strict build.

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

docs/mkdocs/docs/api/basic_json_document/load.md

@@ -91,7 +91,7 @@ additionally linear in the combined length of the text and the decoded strings;
 
 ## Notes
 
-**The `image_check` modes.**
+### `image_check` { #image_check }
 
 ```cpp
 using image_check = detail::view::image_check;