don't auto-run prettier on handbook markdown files
Created by: sqs
We don't want to run prettier to standardize Markdown formatting on handbook Markdown files for the reasons discussed in https://github.com/sourcegraph/about/pull/429#discussion_r353997479.
Many teammates use the prettier VS Code editor extension, which runs on all Markdown files by default. That means their handbok edits include a lot of unrelated prettier edits.
The prettier.requireConfig
VS Code setting to the rescue! It tells prettier not to run on the handbook files (because there is no explicit prettier config file that applies to them). HT @felixfbecker at https://github.com/sourcegraph/about/pull/429#discussion_r354010829