fix(docs): add newlines between prefix/suffix chapters

add newlines between prefix/suffix chapters in SUMMARY.md
to ensure correct extraction by mdbook-i18n-helpers.

Author: hwhsu1231

src/doc/rustc-dev-guide/src/SUMMARY.md

@@ -1,6 +1,7 @@
 # Summary
 
 [Getting Started](./getting-started.md)
+
 [About this guide](./about-this-guide.md)
 
 ---
@@ -230,9 +231,13 @@
 ---
 
 [Appendix A: Background topics](./appendix/background.md)
+
 [Appendix B: Glossary](./appendix/glossary.md)
+
 [Appendix C: Code Index](./appendix/code-index.md)
+
 [Appendix D: Compiler Lecture Series](./appendix/compiler-lecture.md)
+
 [Appendix E: Bibliography](./appendix/bibliography.md)
 
 [Appendix Z: HumorRust](./appendix/humorust.md)