Start the port of the reference to mdBook

This only really moves the files, there's a lot more work coming
in the next commits.

Part of rust-lang#39588.

.gitignore

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+book

src/SUMMARY.md

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+# The Rust Reference
+
+[Introduction](introduction.md)
+
+- [Notation](notation.md)
+    - [Unicode productions](unicode-productions.md)
+    - [String table productions](string-table-productions.md)
+
+- [Lexical structure](lexical-structure.md)
+    - [Input format](input-format.md)
+    - [Identifiers](identifiers.md)
+    - [Comments](comments.md)
+    - [Whitespace](whitespace.md)
+    - [Tokens](tokens.md)
+    - [Paths](paths.md)
+
+- [Macros](macros.md)
+    - [Macros By Example](macros-by-example.md)
+    - [Procedrual Macros](procedural-macros.md)
+
+- [Crates and source files](crates-and-source-files.md)
+
+- [Items and attributes](items-and-attributes.md)
+    - [Items](items.md)
+    - [Visibility and Privacy](visibility-and-privacy.md)
+    - [Attributes](attributes.md)
+
+- [Statements and expressions](statements-and-expressions.md)
+    - [Statements](statements.md)
+    - [Expressions](expressions.md)
+
+- [Type system](type-system.md)
+    - [Types](types.md)
+    - [Subtyping](subtyping.md)
+    - [Type coercions](type-coercions.md)
+
+- [Special traits](special-traits.md)
+    - [The Copy trait](the-copy-trait.md)
+    - [The Sized trait](the-sized-trait.md)
+    - [The Drop trait](the-drop-trait.md)
+    - [The Deref trait](the-deref-trait.md)
+    - [The Send trait](the-send-trait.md)
+    - [The Sync trait](the-sync-trait.md)
+
+- [Memory model](memory-model.md)
+    - [Memory allocation and lifetime](memory-allocation-and-lifetime.md)
+    - [Memory ownership](memory-ownership.md)
+    - [Variables](variables.md)
+
+- [Linkage](linkage.md)
+
+- [Unsafety](unsafety.md)
+    - [Unsafe functions](unsafe-functions.md)
+    - [Unsafe blocks](unsafe-blocks.md)
+    - [Behavior considered undefined](behavior-considered-undefined.md)
+    - [Behavior not considered unsafe](behavior-not-considered-unsafe.md)
+
+[Appendix: Influences](influences.md)