Offline process plugin installation instructions

[ilkka]

I think it would be really nice to have instructions on how to install process plugins in an offline environment. I had lots of success just dropping WASM files in a directory and pointing at those, but then had a spot of trouble with the prettier plugin. Turns out in the end what I had to do was:

• grab their JSON and save it locally
• grab the zip file for my platform, also save locally
• edit the JSON to make the zip file reference a local path
• add the JSON checksum to the main dprint config when it complains about it

Maybe this could be a sidebar in the docs or something. Thanks!