chore: remove unused Syndication dependency, fix README and add infra/docs to Solution Items

[artcava]

Summary

Closes #85

Changes

src/XPoster.csproj

• Removed unused System.ServiceModel.Syndication PackageReference — no usage found anywhere in the codebase (SyndicationFeed, SyndicationItem, etc.)

README.md

• Removed System.ServiceModel.Syndication entry from Technologies → Utilities section
• Fixed workflow badge and CI/CD reference in Deployment section: master_xposterfunction.yml → ci.yml

XPoster.sln

• Added infra Solution Folder (nested under Solution Items) with:
  • infra\README.md
  • infra\main.bicep
  • infra\main.bicepparam
• Added modules Solution Folder (nested under infra) with:
  • infra\modules\function-app.bicep
  • infra\modules\key-vault.bicep
  • infra\modules\monitoring.bicep
  • infra\modules\storage.bicep

ℹ️ GitHub Actions workflows (.github/workflows/*.yml) are intentionally not added as a Solution Folder — Visual Studio 2022 already surfaces them natively via the built-in GitHub Actions node, avoiding duplication in Solution Explorer.

Checklist

• dotnet build passes after PackageReference removal
• README Technologies section no longer references System.ServiceModel.Syndication
• infra and modules folders visible in Visual Studio Solution Explorer
• No duplicate entries for GitHub Actions workflows