Add Papercut SMTP to packages

[mguinness]

Related #940

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• New integration
  • Docs are written
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

[Copilot]

Pull request overview

This PR adds the Papercut SMTP hosting integration to the list of available packages in the README.md, making it discoverable to users of the Aspire Community Toolkit.

Key changes:

• Added Papercut SMTP integration entry to the integrations table with links to documentation and NuGet packages
• Added markdown reference link definitions for the integration's documentation and NuGet package URLs

[Copilot]

The NuGet URL uses www.nuget.org while all other integrations consistently use nuget.org (without www). For consistency with the rest of the file, change to:

[papercutsmtp-nuget]: https://nuget.org/packages/CommunityToolkit.Aspire.Hosting.PapercutSmtp/

Suggested change

	[papercutsmtp-nuget]: https://www.nuget.org/packages/CommunityToolkit.Aspire.Hosting.PapercutSmtp/
	[papercutsmtp-nuget]: https://nuget.org/packages/CommunityToolkit.Aspire.Hosting.PapercutSmtp/

[Copilot]

This line has excessive trailing whitespace before the pipe character (|) separating the first and second columns. The spacing should match other entries in the table (approximately 23 spaces based on line 48 and 50). The current line appears to have significantly more spacing, breaking visual alignment in the markdown table.

Suggested change

| - **Learn More**: [`Hosting.PapercutSmtp`][papercutsmtp-integration-docs] <br /> - Stable 📦: [![CommunityToolkit.Aspire.Hosting.PapercutSmtp][papercutsmtp-shields]][papercutsmtp-nuget] <br /> - Preview 📦: [![CommunityToolkit.Aspire.Hosting.PapercutSmtp][papercutsmtp-shields-preview]][papercutsmtp-nuget-preview] | An Aspire integration leveraging the [Papercut SMTP](https://github.com/ChangemakerStudios/Papercut-SMTP) container. |

| - **Learn More**: [`Hosting.PapercutSmtp`][papercutsmtp-integration-docs] <br /> - Stable 📦: [![CommunityToolkit.Aspire.Hosting.PapercutSmtp][papercutsmtp-shields]][papercutsmtp-nuget] <br /> - Preview 📦: [![CommunityToolkit.Aspire.Hosting.PapercutSmtp][papercutsmtp-shields-preview]][papercutsmtp-nuget-preview] | An Aspire integration leveraging the [Papercut SMTP](https://github.com/ChangemakerStudios/Papercut-SMTP) container. |

[Copilot]

The documentation link uses a GitHub URL pointing to the source directory, while other integrations (like LavinMQ, MailPit, k6) use Microsoft Learn documentation URLs (e.g., https://learn.microsoft.com/dotnet/aspire/community-toolkit/hosting-<integration>). Consider using a consistent pattern:

[papercutsmtp-integration-docs]: https://learn.microsoft.com/dotnet/aspire/community-toolkit/hosting-papercutsmtp

This would be consistent with the MailPit integration above and other hosting integrations.

Suggested change

	[papercutsmtp-integration-docs]: https://github.com/CommunityToolkit/Aspire/tree/main/src/CommunityToolkit.Aspire.Hosting.PapercutSmtp#readme
	[papercutsmtp-integration-docs]: https://learn.microsoft.com/dotnet/aspire/community-toolkit/hosting-papercutsmtp

[mguinness]

@IEvangelist I don't have the technical capability to contribute to the aspire.dev repo. Is there a possibility to provide Markdown to be integrated by an existing contributor?