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/mkdocs/docs/api/basic_json/operator_eq.md

@@ -1,4 +1,4 @@
-# basic_json::operator==
+# <small>nlohmann::basic_json::</small>operator==
 
 ```cpp
 bool operator==(const_reference lhs, const_reference rhs) noexcept;
@@ -85,7 +85,7 @@ Linear.
     }
     ```
 
-## Example
+## Examples
 
 ??? example
 
@@ -101,6 +101,20 @@ Linear.
     --8<-- "examples/operator__equal.output"
     ```
 
+??? example
+
+    The example demonstrates comparing several JSON types against the null pointer (JSON `#!json null`).
+        
+    ```cpp
+    --8<-- "examples/operator__equal__nullptr_t.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/operator__equal__nullptr_t.output"
+    ```
+
 ## Version history
 
 - Added in version 1.0.0.