Start annotating which version of the spec added a thing (#3425)

* Introduce a new "added-in" template and use it on endpoints

* Use "added-in" on schema properties too

* Annotate sections of the spec with their added versions

* Demo of "added-in" on a room version (to be fleshed out)

* Use clearer versioning semantics

* Update and fix validator for Swagger custom properties

* Fix docs

layouts/shortcodes/added-in.html

@@ -0,0 +1,8 @@
+{{ $ver := .Params.v }}
+{{ $this := .Params.this }}
+
+{{ if $this }}
+  <span>**[New in this version]**</span>
+{{ else }}
+  <span>**[Added in `v{{ $ver }}`]**</span>
+{{ end }} {{/* Do not leave an empty line at the end of this file otherwise the inline behaviour breaks. */}}

layouts/shortcodes/definition.html

@@ -37,6 +37,10 @@
 
 <hr/>
 
+{{ if (index $definition "x-addedInMatrixVersion") }}
+  {{ partial "added-in" (dict "v" (index $definition "x-addedInMatrixVersion")) }}
+{{ end }}
+
 {{ $definition.description | markdownify }}
 
 </summary>