[Task]: Add new Python SchemaTransform multi-language documentation

[ahmedabu98]

What needs to happen?

Based on https://docs.google.com/document/d/1_embA3pGwoYG7sbHaYzAkg3hNxjTughhFCY8ThcoK_Q/edit?tab=t.0

Full PR is #33348, but is broken down to separate PRs here:

• [docs] Add new Python SchemaTransform multi-lang guide #33362
  • New guide for SchemaTransform multi-language pipelines
• [docs] Add new Python multi-lang quickstart using the SchemaTransform framework #33360
  • New quick-start
• Python ExternalTransformProvider improvements #33359
  • Improvements to documentation and error message
• Improve existing Python multi-lang SchemaTransform examples #33361
  • Cleaning up the existing example

Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

Issue Components

• Component: Python SDK
• Component: Java SDK
• Component: Go SDK
• Component: Typescript SDK
• Component: IO connector
• Component: Beam YAML
• Component: Beam examples
• Component: Beam playground
• Component: Beam katas
• Component: Website
• Component: Infrastructure
• Component: Spark Runner
• Component: Flink Runner
• Component: Samza Runner
• Component: Twister2 Runner
• Component: Hazelcast Jet Runner
• Component: Google Cloud Dataflow Runner

[ahmedabu98]

@chamikaramj
@robertwb
@Polber
@damondouglas

CC'ing y'all as people who frequently interact with the cross-language interface. I believe we don't have any public documentation on SchemaTransforms yet, so would love some feedback on these new docs and examples before they go out.