Documentation pages

Problem/Motivation

This project has its own documentation pages with content maintained in the code repository, and built via Gitlab Pages and MkDocs. Some of these pages have been reviewed and updated, but others still need expanding and improving. This issue continues the work from #3439644: Review all documentation pages

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/stylelint
jobs/test-only-changes
jobs/phpunit
jobs/phpstan

Original documentation on drupal.org

The project originally had documentation only on this drupal.org page. This page can remain, but some sections and details are duplicated in the project pages. This needs reviewing and stream-lining, because they may be out of date. Reducing duplication is good.

Edited by drupalbot
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