Add DEVELOPER_GUIDE.md for new contributors

[pradeepto]

Summary

• Adds a developer onboarding guide covering the project end to end: architecture, CRD types, controller reconciliation flows, external service integrations (AWS, Docling, embedding API, LangChain), FileStore design, build system, testing with LocalStack, deployment, and troubleshooting.
• Includes a table of contents, code examples, field reference tables, and an FAQ section.

Test plan

• Review file path references for accuracy
• Confirm code examples match current codebase patterns
• Check controller flow descriptions against current logic

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a detailed developer guide to centralize project knowledge and improve the onboarding experience for new contributors. It provides a comprehensive overview of the operator's architecture, API types, and operational workflows, serving as a single source of truth for development, testing, and deployment processes.

Highlights

• Documentation: Added a comprehensive DEVELOPER_GUIDE.md to streamline onboarding for new contributors.
• Technical Content: The guide details the project architecture, CRD types, controller reconciliation flows, and external service integrations.
• Developer Experience: Includes build system instructions, testing guidelines with LocalStack, and an FAQ section to assist with local development and troubleshooting.

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[gemini-code-assist]

Code Review

This pull request introduces a comprehensive Developer Guide (DEVELOPER_GUIDE.md) for the Unstructured Data Controller, detailing its architecture, data models, business logic, external integrations, and development setup. The review feedback points out inconsistencies in the documented Go versions across different sections and an incorrect reference to a non-existent version of golangci-lint.

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[gemini-code-assist]

There are several inconsistencies regarding the required Go version throughout the developer guide:

• Section 1 (line 37): Specifies Go 1.25+
• Section 9 (line 691): Specifies Go 1.25.0+
• Dev Container (line 737): Uses Go 1.24 (1-1.24-bookworm)
• Section 10 (line 802): Mentions ubi9/go-toolset:1.26 (Go 1.26)

Please align these references to the actual minimum Go version required by the project (e.g., Go 1.24) to avoid confusion for new contributors.

[gemini-code-assist]

The guide references golangci-lint v2.10.0 (line 697) and golangci-lint v2 (line 864). However, golangci-lint is currently on version v1.x (e.g., v1.64.0). There is no v2 release of golangci-lint yet.

Please correct these references to the appropriate v1.x version used by the project.

[PuneetPunamiya]

We might have to update this with the latest API changes

[PuneetPunamiya]

Wanted to understand the goal behind adding this readme

• Is it for just giving the complete understanding of the project
• Do we need to add steps for running the project as well (docling, localstack, etc)

[PuneetPunamiya]

We do not have sqsInformer CRD now