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/to_string.cpp

@@ -0,0 +1,20 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+using std::to_string;
+
+int main()
+{
+    // create values
+    json j = {{"one", 1}, {"two", 2}};
+    int i = 42;
+
+    // use ADL to select best to_string function
+    auto j_str = to_string(j);  // calling nlohmann::to_string
+    auto i_str = to_string(i);  // calling std::to_string
+
+    // serialize without indentation
+    std::cout << j_str << "\n\n"
+              << i_str << std::endl;
+}