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

@@ -0,0 +1,41 @@
+# <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.