docs(web): add playground PR examples to documentation

[parse]

Summary

• Expanded the "Try the example project" section in getting-started with descriptions of all three playground PRs (erode-app/playground#1 undeclared dep with LikeC4, erode-app/playground#2 same with Structurizr, erode-app/playground#3 new component + model update flow)
• Replaced generic "See it in action" tips in the LikeC4 and Structurizr model pages with concrete descriptions of what each PR demonstrates
• Added a "See it in action" tip to the GitHub Actions model update PRs section, linking erode-app/playground#3 and its resulting model update PR

Test plan

• npm run format:check passes
• npm run lint:md passes
• Verify rendered pages with npm run dev:web