Add a README for every example and standardize on the Estuary name

[danthelion]

Summary

Adds or rewrites a technical README.md for every example in the repo and rebuilds the root README.md as a categorized index. Also standardizes the product name as Estuary throughout (no "Estuary Flow" / bare "Flow").

Two commits:

1. docs: add and refresh README for every example — documentation only.
2. fix: correct latent bugs in example projects — small code fixes surfaced while writing the docs, plus README sync.

READMEs

• Covered all 29 top-level examples + the 4 Python-derivation subprojects, and rebuilt the root index (Database CDC Captures / Materializations & Destinations / Derivations & Transformations / Real-Time RAG & AI / Streaming, Lakehouse & Stream Processing / Demos, Workshops & Webinars). All index links verified to resolve.
• 8 brand-new READMEs: dekaf-python, estuary-demo-movies, mongodb-tinybird-clickstream, pyiceberg-aws-glue, python-derivations, and shipments-{ai,joins,stateless}.
• Replaced placeholder stubs (the "??? / Profit!" and bare blog-link READMEs); preserved existing strong content (e.g. sqlserver-cdc-capture, oracle-capture, kafka-capture, shipments-datagen, the shipments-stateful Mermaid diagram, the hands-on-lab steps).
• Each README documents the data flow in Estuary terms (capture → collection → materialization/derivation), what's included, prerequisites, copy-pasteable setup, and how to configure the capture/materialization via the dashboard or flowctl, with links to the relevant connector docs. Values (ports, users, tables, connector names) were verified against the actual docker-compose, init.sql, flow.yaml, and datagen files.

Naming

All platform references now say Estuary. flowctl, flow.yaml / .flow.py, and flow_capture / flow_publication / flow_watermarks identifiers are left untouched.

Fixes (commit 2)

Example	Fix
dekaf-kcat	consume.sh bootstrap host dekaf.fly.dev:9092 (retired) → dekaf.estuary-data.com:9092
postgres-cloudsql-simple-capture	datagen/requirements.txt missing SQLAlchemy + python-dotenv (imported by datagen.py)
streaming-lakehouse-iceberg-duckdb	requirements.txt missing python-dotenv (imported by main.py)
python-derivations/shipments-stateful	derivation used 'Out for Delivery'; generator emits 'Out For Delivery' (status never matched)
shipments_eta	docker-compose.yml hardcoded dead localhost/mongo values → pass MongoDB env vars through
shipments_eta	eta.sql arrayJoin(s.delays_reason, …) → arrayStringConcat(s.delays__reason, …) (matches the Data Source column)

Left as-is (flag for review)

shipments_eta/datagen/datagen.py and tinybird/shipments.datasource still contain personal-looking values (dani2, a specific Atlas cluster host, KAFKA_CONNECTION_NAME 'Estuary Flow', the Dani/shipments-demo prefix). Left untouched since they're functional config; happy to scrub them in a follow-up if desired.