Auto merge of #19353 - icorderi:docs/grammar, r=steveklabnik

Original [issue](#19278) that inspired this patch.

The [reference.md] has evolved past simple grammatical constructs, and it serves a different purpose. 
The intent for the proposed _grammar.md_ is to hold **only** the official reference for the language grammar. This document would keep track of grammatical changes to the language over time, facilitate discussions over proposed changes to the existing grammar, and serve as basis for building parsers by third-parties (IDE's, GitHub linguist, CodeMirror, etc.). 

The current state of the PR contains all the grammars that were available in [reference.md] and nothing else. 
There are still a lot of missing pieces that weren't available. The following are just a few of the definitions missing:
- [Functions](https://github.com/icorderi/rust/blob/docs/grammar/src/doc/grammar.md#functions)
- [Structures](https://github.com/icorderi/rust/blob/docs/grammar/src/doc/grammar.md#structures)
- [Traits](https://github.com/icorderi/rust/blob/docs/grammar/src/doc/grammar.md#traits)
- [Implementations](https://github.com/icorderi/rust/blob/docs/grammar/src/doc/grammar.md#implementations)
- [Operators](https://github.com/icorderi/rust/blob/docs/grammar/src/doc/grammar.md#unary-operator-expressions)
- [Statements](https://github.com/icorderi/rust/blob/docs/grammar/src/doc/grammar.md#statements)
- [Expressions](https://github.com/icorderi/rust/blob/docs/grammar/src/doc/grammar.md#expressions)

[reference.md]: https://github.com/rust-lang/rust/blob/master/src/doc/reference.md

We need help from people familiar with those grammatical constructs to fill in the missing pieces.