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/examples/std_swap.output

@@ -0,0 +1,2 @@
+j1 = {"one":1,"two":2} | j2 = [1,2,4,8,16]
+j1 = [1,2,4,8,16] | j2 = {"one":1,"two":2}