chore: update .gitignore and remove obsolete assets

- Add build artifact directories and backup files to .gitignore
  (jattach build, kernel build, *.back).
- Remove old WeChat group keeper image and SIGCOMM 2023 paper PDF
  that are no longer referenced in documentation.
- Update rebuilt libjattach.a static library.

Author: ioki-smore

.gitignore

@@ -34,3 +34,7 @@ logs/
 .vscode/
 .zed/
 ignored/
+src/collectors/ebpf/legacy/libs/jattach/build/
+src/collectors/ebpf/legacy/kernel/build/
+*.back
+