Add CSI resource pages and navigation

[Ve Sharma (VeVarunSharma)]

Summary

Expand the GitHub @ CSI site into four focused, README-backed sections:

• Resource Hub
• CSI Build Days
• CSI Pricing
• Usage-Based Billing

What changed

• Added dedicated CSI Build Days and CSI Pricing pages
• Marked Developer and Product tracks as completed
• Added QA and DevSecOps registration links
• Added four previous session recordings with clear Microsoft sign-in labels
• Added the public Build Days recording playlist as an anonymous-access alternative
• Consolidated UBB guidance and resources
• Simplified the Resource Hub into concise section summaries
• Improved mobile tabs, theme persistence, deep-link navigation, and Markdown rendering
• Included the updated discount verification image and refined homepage logo sizing
• Added a pull-request CI workflow that runs npm ci and npm run build

Validation

• npm ci
• npm run build
• Verified all four routes on desktop and mobile
• Verified light, dark, and system theme behavior
• Verified completed track statuses, registration links, recording labels, deep links, and pricing imagery

Access note

Individual SharePoint recordings require Microsoft sign-in. The public recording playlist is provided as an anonymous-access alternative.

Repository setup after merge

After the first PR CI run, add Build site as a required status check for main in the repository ruleset.