Book: Restructure Dev chapter

Group everything that has something to do with Clippy development under
the "Development" chapter, so that Clippy users can't get confused.

book/src/SUMMARY.md

@@ -13,11 +13,11 @@
     - [Basics](development/basics.md)
     - [Adding Lints](development/adding_lints.md)
     - [Common Tools](development/common_tools_writing_lints.md)
-- [Infrastructure](infrastructure/README.md)
-    - [Syncing changes between Clippy and rust-lang/rust](infrastructure/sync.md)
-    - [Backporting Changes](infrastructure/backport.md)
-    - [Updating the Changelog](infrastructure/changelog_update.md)
-    - [Release a New Version](infrastructure/release.md)
-    - [The Clippy Book](infrastructure/book.md)
-- [Roadmap](roadmap/README.md)
-    - [2021](roadmap/2021.md)
+    - [Infrastructure](development/infrastructure/README.md)
+        - [Syncing changes between Clippy and rust-lang/rust](development/infrastructure/sync.md)
+        - [Backporting Changes](development/infrastructure/backport.md)
+        - [Updating the Changelog](development/infrastructure/changelog_update.md)
+        - [Release a New Version](development/infrastructure/release.md)
+        - [The Clippy Book](development/infrastructure/book.md)
+    - [Proposals](development/proposals/README.md)
+        - [Roadmap 2021](development/proposals/roadmap-2021.md)

book/src/development/proposals/README.md

@@ -0,0 +1 @@
+# Proposals