[LangRef] Reduce depth of table of contents

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: 08013345

Differential Revision: https://reviews.llvm.org/D130635