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/adl_serializer/from_json.md

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+# <small>nlohmann::adl_serializer::</small>from_json
+
+```cpp
+// (1)
+template<typename BasicJsonType, typename TargetType = ValueType>
+static auto from_json(BasicJsonType && j, TargetType& val) noexcept(
+    noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val)))
+-> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), val), void())
+
+// (2)
+template<typename BasicJsonType, typename TargetType = ValueType>
+static auto from_json(BasicJsonType && j) noexcept(
+noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {})))
+-> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {}))
+```
+
+This function is usually called by the [`get()`](../basic_json/get.md) function of the
+[basic_json](../basic_json) class (either explicit or via conversion operators).
+
+1. This function is chosen for default-constructible value types.
+2. This function is chosen for value types which are not default-constructible.
+
+## Parameters
+
+`j` (in)
+:   JSON value to read from
+
+`val` (out)
+:   value to write to
+
+## Return value
+
+Copy of the JSON value, converted to `ValueType`
+
+!!! note
+
+    This documentation page is a stub.
+
+## Version history
+
+- Added in version 2.1.0.

doc/mkdocs/docs/api/adl_serializer/index.md

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+# <small>nlohmann::</small>adl_serializer
+
+```cpp
+template<typename, typename>
+struct adl_serializer;
+```
+
+Serializer that uses ADL ([Argument-Dependent Lookup](https://en.cppreference.com/w/cpp/language/adl)) to choose
+`to_json`/`from_json` functions from the types' namespaces.
+
+It is implemented similar to
+
+```cpp
+template<typename ValueType>
+struct adl_serializer {
+    template<typename BasicJsonType>
+    static void to_json(BasicJsonType& j, const T& value) {
+        // calls the "to_json" method in T's namespace
+    }
+
+    template<typename BasicJsonType>
+    static void from_json(const BasicJsonType& j, T& value) {
+        // same thing, but with the "from_json" method
+    }
+};
+```
+
+## Member functions
+
+- [**from_json**](from_json.md) - convert a JSON value to any value type
+- [**to_json**](to_json.md) - convert any value type to a JSON value
+
+## Version history
+
+- Added in version 2.1.0.

doc/mkdocs/docs/api/adl_serializer/to_json.md

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+# <small>nlohmann::adl_serializer::</small>to_json
+
+```cpp
+template<typename BasicJsonType, typename TargetType = ValueType>
+static auto to_json(BasicJsonType& j, TargetType && val) noexcept(
+    noexcept(::nlohmann::to_json(j, std::forward<TargetType>(val))))
+-> decltype(::nlohmann::to_json(j, std::forward<TargetType>(val)), void())
+```
+
+This function is usually called by the constructors of the [basic_json](../basic_json) class.
+
+## Parameters
+
+`j` (out)
+:   JSON value to write to
+
+`val` (in)
+:   value to read from
+
+!!! note
+
+    This documentation page is a stub.
+
+## Version history
+
+- Added in version 2.1.0.