Document how to setup macOS (and other platforms) for Metanorma development

[ronaldtse]

We used to recommend https://github.com/metanorma/metanorma-macos-setup/ but it is now archived/obsolete due to other user setup options.

However, we are now missing appropriate documentation for developers who wish to develop with Metanorma and/or use other Ruby tools (e.g. stepmod-utils) with Metanorma.

This task is to provide such documentation.

[ronaldtse]

So far I've been using something like this:

# Install Xcode and associated development tools
xcode-select —install

# Install GCC and development tools
brew install autoconf gcc make

# Install rbenv (the default macOS Ruby is used by system and can be tricky with installs/uninstalls)
brew install rbenv

# Install Ruby 2.7.4
rbenv install 2.7.4

# Install stepmod-utils
gem install stepmod-utils

However this set of commands may look different for M1:
rbenv/ruby-build#1691

[CAMOBAP]

Done