Add data ingestion quickstart for processing custom data

[Copilot]

Summary

Adds quickstart documentation for Microsoft.Extensions.DataIngestion library, demonstrating complete ETL pipeline for RAG scenarios.

Contributes to #50534

Changes

Documentation

• New quickstart: docs/ai/quickstarts/process-data.md
  • Document reading with MarkdownReader
  • AI-powered enrichment (image alt-text, summaries)
  • Semantic chunking with embedding-based similarity
  • Vector storage using SQLite
  • Interactive semantic search
• Dual-platform support via zone pivots (OpenAI/Azure OpenAI)
• Added to "Chat with your data (RAG)" section in TOC

Code Snippets

• Complete compilable C# projects for both platforms
• Demonstrates pipeline composition: reader → enricher → chunker → writer
• Includes sample data and region markers for doc references

// Compose data ingestion pipeline
using IngestionPipeline<string> pipeline = new(reader, chunker, writer, loggerFactory)
{
    DocumentProcessors = { imageAlternativeTextEnricher },
    ChunkProcessors = { summaryEnricher }
};

await foreach (var result in pipeline.ProcessAsync(new DirectoryInfo("./data"), searchPattern: "*.md"))
{
    Console.WriteLine($"Completed processing '{result.DocumentId}'. Succeeded: '{result.Succeeded}'.");
}

Based on sample from https://github.com/luisquintanilla/DataIngestion and blog announcement at https://devblogs.microsoft.com/dotnet/introducing-data-ingestion-building-blocks-preview/

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• devblogs.microsoft.com
  • Triggering command: /usr/bin/curl curl -s REDACTED (dns block)
  • Triggering command: /usr/bin/wget wget -q -O /tmp/blog.html REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

Add a quickstart or tutorial for processing custom data based on the data ingestion sample described in https://devblogs.microsoft.com/dotnet/introducing-data-ingestion-building-blocks-preview/. It should live under the "Chat with your data" section of the AI TOC and be titled "Process data" or something similar. This task contributes to #50534.

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Internal previews

📄 File	🔗 Preview link
docs/ai/quickstarts/process-data.md	Process custom data for AI applications
docs/ai/quickstarts/snippets/process-data/data/sample.md	docs/ai/quickstarts/snippets/process-data/data/sample
docs/ai/quickstarts/structured-output.md	Request a response with structured output
docs/ai/quickstarts/text-to-image.md	Quickstart - Generate images from text using AI
docs/ai/toc.yml	docs/ai/toc