Update @antora/assembler to v1.0.0-beta.20 (docs-site) (#5521)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package-lock.json

@@ -8,9 +8,9 @@
   },
   "dependencies": {
     "@antora/collector-extension": "1.0.3",
-    "@antora/html-single-extension": "1.0.0-beta.19",
+    "@antora/html-single-extension": "1.0.0-beta.20",
     "@antora/lunr-extension": "1.0.0-alpha.13",
-    "@antora/pdf-extension": "1.0.0-beta.19",
+    "@antora/pdf-extension": "1.0.0-beta.20",
     "@asciidoctor/tabs": "1.0.0-beta.6",
     "@springio/antora-extensions": "1.14.7",
     "@springio/antora-xref-extension": "1.0.0-alpha.5",