Add Spring REST Docs icon for IntelliJ IDEA

- Add an SVG project icon converted from the official PNG logo
  (source: https://spring.io/img/projects/spring-restdocs.png)
- Update .gitignore to exclude all .idea files except icon.svg

Signed-off-by: Seonghun Jeong <tjdgns5506@gmail.com>

See gh-961

.gitignore

@@ -6,5 +6,6 @@ bin
 build
 !buildSrc/src/main/groovy/org/springframework/restdocs/build/
 target
-.idea
+.idea/*
+!.idea/icon.svg
 *.iml