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/flatten.md

@@ -1,4 +1,4 @@
-# basic_json::flatten
+# <small>nlohmann::basic_json::</small>flatten
 
 ```cpp
 basic_json flatten() const;
@@ -25,7 +25,7 @@ Linear in the size the JSON value.
 Empty objects and arrays are flattened to `#!json null` and will not be reconstructed correctly by the
 [`unflatten()`](unflatten.md) function.
 
-## Example
+## Examples
 
 ??? example
 
@@ -41,6 +41,10 @@ Empty objects and arrays are flattened to `#!json null` and will not be reconstr
     --8<-- "examples/flatten.output"
     ```
 
+## See also
+
+- [unflatten](unflatten.md) the reverse function
+
 ## Version history
 
 - Added in version 2.0.0.