📚 Update HAR instructions

Zendesk has a very good instructive article about capturing HARs from different browsers: https://support.zendesk.com/hc/en-us/articles/204410413-Generating-a-HAR-file-for-troubleshooting

It's dated though (Firefox has this built-in, and IE is no longer a thing, there's Edge now).

I'd like to redo this with modern browsers, and then publish it somewhere….