Improve the documentation (installing of pdftoppm), particularly of macOS

On http://mirrors.ctan.org/support/ppmcheckpdf/ppmcheckpdf.pdf the documentation show how to install pdftoppm on Windows and Ubuntu/Debian Linux.

For Linux and macOS, there is an alternate way, with the homebrew package manager (not a package in LaTeX sense). The homepage is at https://brew.sh.

As the steps here are (too) detailed and long, perhaps you can only say in the manual:

install the homebrew package manager, then run brew install poppler

1. Install the homebrew package manager

Paste /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)" on a Terminal (Mac) or a shell prompt (Linux)

Notes:

On macOS, the first step will install "Command Line Tools for Xcode" before installing the brew command. This will take 1 or 2 minutes on a fast connexion.
I don't know if this is the same on Linux, but on macOS, a message is shown asking us to run two last commands before the brew command is available.
- (echo; echo 'eval "$(/opt/homebrew/bin/brew shellenv)"') >> /Users/__yourUserName__/.zprofile where __yourUserName__ is replaced with your real userName
- eval "$(/opt/homebrew/bin/brew shellenv)"

2. Install `poppler` with the`brew` command

Paste brew install poppler on a Terminal or a prompt shell.

Note: This will install a lot of dependencies. This is expected.

lvjr / ppmcheckpdf