You can clone the website repository to your local machine using git:
git clone https://github.com/node-red/node-red.github.io.git
That will create a directory called node-red.github.io that contains all the source of the website.
That'll get you the docs in their raw markdown format - fairly readable plain text.
If you wanted to go above and beyond, you can install Jekyll and then...
cd node-red.github.io.git
jekyll serve
That will build the documentation and start a webserver at http://127.0.0.1:4000 - open that in a browser and you'll have the node-red website locally.
Word of warning - to install jekyll you'll need to have a working install of ruby and depending on your OS etc, that may just be one rabbit hole too many.
On that same principle though, you can also grab the cookbook:
git clone https://github.com/node-red/cookbook.nodered.org.git