Move spring-framework.png to src/docs folder

Since the spring-framework.png file is not used in the generated
reference manual, this commit moves the spring-framework.png file from
the src/docs/asciidoc/images folder up to the src/docs folder.

README.md

@@ -1,4 +1,4 @@
-# <img src="src/docs/asciidoc/images/spring-framework.png" width="80" height="80"> Spring Framework [![Build Status](https://ci.spring.io/api/v1/teams/spring-framework/pipelines/spring-framework-5.3.x/jobs/build/badge)](https://ci.spring.io/teams/spring-framework/pipelines/spring-framework-5.3.x?groups=Build")
+# <img src="src/docs/spring-framework.png" width="80" height="80"> Spring Framework [![Build Status](https://ci.spring.io/api/v1/teams/spring-framework/pipelines/spring-framework-5.3.x/jobs/build/badge)](https://ci.spring.io/teams/spring-framework/pipelines/spring-framework-5.3.x?groups=Build")
 
 This is the home of the Spring Framework: the foundation for all [Spring projects](https://spring.io/projects). Collectively the Spring Framework and the family of Spring projects are often referred to simply as "Spring".