docs: update architecture overview to Airflow 3 architecture

[JH-A-Kim]

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Docs-only change

Description

This PR aims at updating the page: Architecture Overview from Airflow 2 architecture to Airflow 3 architecture to properly align with the current architecture.

It specifically targets a few core sections:

• Required components
• Basic Airflow Deployment
• Distributed Airflow Architecture
• Separate Dag processing Architecture

It updates them by doing the following changes:

• Replace webserver references with API server
• Introduce Dag bundles as a required component
• Correct Dag processor as required in all Airflow 3 deployments
• Update Dag file sync description
• Remove stale "separate Dag processing"
• Add TODO for remaining AIP-72 diagram work.
• Replaced metadata db access connections in legend
• Fix claim that the scheduler reads Dag files directly
• Add Task SDK as required component of minimal installation

Sources used for update

• https://airflow.apache.org/docs/apache-airflow/stable/release_notes.html#dag-versioning-aip-66
• https://airflow.apache.org/docs/apache-airflow/stable/release_notes.html#airflow-3-0-0-2025-04-22
• https://airflow.apache.org/docs/apache-airflow/stable/release_notes.html#task-execution-api-task-sdk-aip-72
• https://airflow.apache.org/docs/apache-airflow/stable/release_notes.html#standalone-dag-processor-required
• https://airflow.apache.org/docs/apache-airflow/stable/release_notes.html#restricted-metadata-database-access
• https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=294816356

Out of Scope

• Diagram image updates require separate work to meet AIP-72 specification. This is tracked in the TODO comment added to the RST.

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: Claude Sonnet 4.6 following the guidelines

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[boring-cyborg]

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