Make definition anchors more unique (#1339)

Stick a `definition-` on the front of the autogenerated anchors for definition
blocks.

This solves a problem where, for example,
https://spec.matrix.org/unstable/application-service-api/#registration could
refer to either the "Registration" section or the `Registration` definition
therein.

(These anchors are relatively recent: they were added in #1191.

changelogs/internal/newsfragments/1339.clarification

@@ -0,0 +1 @@
+Make definition anchors more unique.

layouts/shortcodes/definition.html

@@ -30,7 +30,7 @@
 {{ $definition = partial "json-schema/resolve-refs" (dict "schema" $definition "path" $path) }}
 {{ $definition = partial "json-schema/resolve-allof" $definition }}
 
-<section class="rendered-data definition" id="{{ anchorize $definition.title }}">
+<section class="rendered-data definition" id="definition-{{ anchorize $definition.title }}">
 
 <details {{ if not $compact }}open{{ end }}>
 <summary>