Gray out appendices that don't have any content yet

second-edition/src/SUMMARY.md

@@ -114,8 +114,8 @@
 - [Appendix](appendix-00.md)
     - [A - Keywords](appendix-01-keywords.md)
     - [B - Operators](appendix-02-operators.md)
-    - [C - Derivable Traits](appendix-03-derivable-traits.md)
-    - [D - Nightly Rust](appendix-04-nightly-rust.md)
-    - [E - Macros](appendix-05-macros.md)
-    - [F - Translations](appendix-06-translation.md)
+    - [C - Derivable Traits]()
+    - [D - Nightly Rust]()
+    - [E - Macros]()
+    - [F - Translations]()
     - [G - Newest Features](appendix-07-newest-features.md)

second-edition/src/appendix-03-derivable-traits.md

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-## Appendix C: Derivable Traits

second-edition/src/appendix-04-nightly-rust.md

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-## Appendix D: Nightly Rust

second-edition/src/appendix-05-macros.md

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-## Appendix E: Macros
-
-## Basics of writing your own macros
-
-## Macros are changing, go see X for more info