Consolidate documentation (#3071)

* 🔥 consolidate documentation
* ♻️ overwork std specializations
* 🚚 move images files to mkdocs
* ♻️ fix URLs
* 🔧 tweak MkDocs configuration
* 🔧 add namespaces
* 📝 document deprecations
* 📝 document documentation generation
* 🚸 improve search
* 🚸 add examples
* 🚧 start adding documentation for macros
* 📝 add note for https://github.com/nlohmann/json/issues/874#issuecomment-1001699139
* 📝 overwork example handling
* 📝 fix Markdown tables

doc/examples/std_hash.cpp

@@ -0,0 +1,18 @@
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    std::cout << "hash(null) = " << std::hash<json> {}(json(nullptr)) << '\n'
+              << "hash(false) = " << std::hash<json> {}(json(false)) << '\n'
+              << "hash(0) = " << std::hash<json> {}(json(0)) << '\n'
+              << "hash(0U) = " << std::hash<json> {}(json(0U)) << '\n'
+              << "hash(\"\") = " << std::hash<json> {}(json("")) << '\n'
+              << "hash({}) = " << std::hash<json> {}(json::object()) << '\n'
+              << "hash([]) = " << std::hash<json> {}(json::array()) << '\n'
+              << "hash({\"hello\": \"world\"}) = " << std::hash<json> {}("{\"hello\": \"world\"}"_json)
+              << std::endl;
+}