Review all documentation pages

Migrated issue

Reported by: jonathan1055

Related to !244 (merged) !202 (merged) !186 (merged) !181 (merged)

Problem/Motivation

This project's documentation pages were gathered together fairly quickly here and published here

Since then, some pages have had a thorough review and overhaul. Others are still in their initial state and need to be reviewed and updated.

Pages that have been reviewed and updated

index

info/testing MRs
info/templates-version
info/common/#viewing-environment-variables
info/drupal7
info/variants

jobs/cspell
jobs/eslint
jobs/test-only-changes
jobs/phpunit
jobs/phpstan