feat(curriculum): CSS Box Model project (#42182)

* feat(curriculum): add initial steps

Add the initial step files for this project.

* feat(curriculum): write test texts

Add some test text (tests coming soon).

* feat: tests for HTML bits

Add tests for the HTML bits - CSS holding off until parser lands.

* feat: CSS tests

New CSS parser works great!

* fix: link href

Fix the href in the boilerplate.

* chore: apply suggestions from code review

Co-authored-by: gikf <60067306+gikf@users.noreply.github.com>

* fix: clarify link element

* fix: optional chaining

* fix: remove unused meta props

* fix: update helper and fix tests

* chore: apply tom's review suggestions

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

* chore: no console logs

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

* feat: intro

Add introductory text, tweak instructions a bit.

Co-authored-by: gikf <60067306+gikf@users.noreply.github.com>
Co-authored-by: Tom <20648924+moT01@users.noreply.github.com>
Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>

client/utils/help-category-map.json

@@ -6,6 +6,7 @@
   "responsive-web-design-principles": "HTML-CSS",
   "css-flexbox": "HTML-CSS",
   "css-grid": "HTML-CSS",
+  "css-box-model": "HTML-CSS",
   "css-picasso-painting": "HTML-CSS",
   "responsive-web-design-projects": "HTML-CSS",
   "basic-javascript": "JavaScript",