metadata: Change section anchor style

Set `idprefix` and `idseparator` so Asciidoctor emits IDs like
`#repository-identity` instead of `#_repository_identity`.

This applies to every RIP that includes `_metadata.adoc`,
so the convention only needs to be configured in this file.

_metadata.adoc

@@ -1,3 +1,9 @@
+// Use anchors of the form `#foo-bar`, i.e., replacing whitespace with "-",
+// instead of instead of AsciiDoc's default `#_snake_case` (replacing
+// whitespace with "_" and prefixing with "_").
+:idprefix:
+:idseparator: -
+
 [horizontal]
 RIP:: {rip}
 ifdef::discussion[Discussion:: {discussion}]