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_ltlt.md

@@ -0,0 +1,62 @@
+# operator<<(basic_json)
+
+```cpp
+std::ostream& operator<<(std::ostream& o, const basic_json& j);
+```
+
+Serialize the given JSON value `j` to the output stream `o`. The JSON value will be serialized using the
+[`dump`](dump.md) member function.
+
+- The indentation of the output can be controlled with the member variable `width` of the output stream `o`. For
+  instance, using the manipulator `std::setw(4)` on `o` sets the indentation level to `4` and the serialization result
+  is the same as calling `dump(4)`.
+- The indentation character can be controlled with the member variable `fill` of the output stream `o`. For instance,
+  the manipulator `std::setfill('\\t')` sets indentation to use a tab character rather than the default space character.
+
+## Parameters
+
+`o` (in, out)
+:   stream to serialize to
+
+`j` (in)
+:   JSON value to serialize
+
+## Return value
+
+the stream `o`
+
+## Exceptions
+
+Throws [`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316) if a string stored inside the JSON value
+is not UTF-8 encoded. Note that unlike the [`dump`](dump.md) member functions, no `error_handler` can be set.
+
+## Complexity
+
+Linear.
+
+## Examples
+
+??? example
+
+    The example below shows the serialization with different parameters to `width` to adjust the indentation level.
+        
+    ```cpp
+    --8<-- "examples/operator_serialize.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/operator_serialize.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0
+- Support for indentation character added in version 3.0.0.
+
+!!! warning "Deprecation"
+
+    This function replaces function `#!cpp std::ostream& operator>>(const basic_json& j, std::ostream& o)` which has
+    been deprecated in version 3.0.0. It will be removed in version 4.0.0. Please replace calls like `#!cpp j >> o;`
+    with `#!cpp o << j;`.