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/json_sax/key.md

@@ -0,0 +1,24 @@
+# <small>nlohmann::json_sax::</small>key
+
+```cpp
+virtual bool key(string_t& val) = 0;
+```
+
+An object key was read.
+
+## Parameters
+
+`val` (in)
+:   object key
+
+## Return value
+
+Whether parsing should proceed.
+
+## Notes
+
+It is safe to move the passed object key value.
+
+## Version history
+
+- Added in version 3.2.0.