[SVC-2569] Add CI check for running `prettier` on all Markdown docs

[NathanFlurry]

Motivation

Validate that our documentation is consistent between all code styles.

Requirements

• Update shell.nix to include the Prettier CLI
• Create a new CI workflow named format.ymlfor this
• Use prettier --check on all Markdown files:
  • docs/
  • errors/
  • README.md

_{From SyncLinear.com | SVC-2569}

[NathanFlurry]

Out of sync with Linear