[LangRef] Reduce depth of table of contents

[foad]

Summary

The table of contents in the HTML version of this doc takes up 25 pages
(in my browser, on my 4K monitor) and is too long for me to navigate
comfortably. And most of it is irrelevant detail like this:

• Bitwise Binary Operations
  • 'shl' Instruction
    • Syntax:
    • Overview:
    • Arguments:
    • Semantics:
    • Example:
  • 'lshr' Instruction
    • Syntax:
    • Overview:
    • Arguments:
    • Semantics:
    • Example:

Reducing the contents depth from 4 to 3 removes most of this detail,
leaving just a list of instructions, which only takes up 7 pages and I
find it much easier to navigate.

Incidentally the depth was set to 3 when this document was first
converted to reST and was only increased to 4 in what looks like an
accidental change: 080133453bae62a43466e881ffc03fb7cffd7288

[aaron.ballman]

LGTM, I think this balances readability a bit better this way, thanks!

llvm/docs/LangRef.rst

 ==============================
 LLVM Language Reference Manual
 ==============================
 
 .. contents::
    :local:
-   :depth: 4
+   :depth: 3
 
 Abstract
 ========
 
 This document is a reference manual for the LLVM assembly language. LLVM
 is a Static Single Assignment (SSA) based representation that provides
 type safety, low-level operations, flexibility, and the capability of
 representing 'all' high-level languages cleanly. It is the common code