Change external asciidoctor tool to embeded mode (#8677)

Ref #8676

* add the `-e` flag for embedded mode
* add a level offset to include the title of the document

docs/content/doc/advanced/external-renderers.en-us.md

 [markup.asciidoc]
 ENABLED = true
 FILE_EXTENSIONS = .adoc,.asciidoc
-RENDER_COMMAND = "asciidoctor --out-file=- -"
+RENDER_COMMAND = "asciidoctor -e -a leveloffset=-1 --out-file=- -"
 ; Input is not a standard input but a file
 IS_INPUT_FILE = false