feat: css photo gallery (#43602)

* feat: initial infra

* feat: create steps

* feat: prototype tests

Haven't tested locally yet :)

* feat: complete tests

* feat: move image size step

* chore: apply shaun's review suggestions

Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>

* chore: apply kris' review suggestions

Co-authored-by: Kristofer Koishigawa <scissorsneedfoodtoo@gmail.com>

* fix: index.md linting

* chore: steps are parts in disguise

* chore: apply tom's review suggestions

Co-authored-by: Tom <20648924+moT01@users.noreply.github.com>

* fix: colon to period

Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>
Co-authored-by: Kristofer Koishigawa <scissorsneedfoodtoo@gmail.com>
Co-authored-by: Tom <20648924+moT01@users.noreply.github.com>

client/src/pages/learn/responsive-web-design/css-photo-gallery/index.md

@@ -0,0 +1,10 @@
+---
+title: Introduction to the CSS Photo Gallery
+block: css-photo-gallery
+superBlock: Responsive Web Design
+isBeta: true
+---
+
+## Introduction to the CSS Photo Gallery
+
+This is a test for the new project-based curriculum.