fix(docs): move banner into docs/ to restore root allowlist

#20 landed cicd-squabbler.png at the repository root, which
check-root-shape.sh rejects: the allowlist admits root entries only
where the tooling requires them, and an AsciiDoc banner does not.
main is currently red on Estate Rules as a result.

docs/ is already allow-listed; the README reference moves with it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

README.adoc

@@ -4,7 +4,7 @@
 :toc: preamble
 :icons: font
 
-image::cicd-squabbler.png[CICD Squabbler Banner,width="100%",link=""]
+image::docs/cicd-squabbler.png[CICD Squabbler Banner,width="100%",link=""]
 
 image:https://img.shields.io/badge/License-MPL_2.0-blue.svg[code: MPL-2.0]
 image:https://img.shields.io/badge/docs-CC--BY--SA--4.0-green.svg[docs: CC-BY-SA-4.0]