docs: write about Rust SDK in the Programming Guide

[laysakura]

Fixes: #6

(I'll try to write the whole contents of the Programming Guide in this PR)

How to check the doc

(Assuming M1 Mac)

DOCKER_DEFAULT_PLATFORM=linux/amd64 ./gradlew :website:serveWebsite
# => access to http://localhost:1313/documentation/programming-guide/

Review status

• (1) Overview
  • [ ] review ready
  • [ ] initial review
  • [ ] working example
• (2) Creating a pipeline
  • [ ] review ready
  • [ ] initial review
  • [ ] working example
• (3) PCollections
  • [x] review ready
  • [ ] initial review
  • [ ] working example
• (4.1) Applying transforms
  • [x] review ready
  • [ ] initial review
  • [ ] working example
• (4.2) Core Beam transforms
  • [x] review ready
  • [ ] initial review
  • [ ] working example
• (7) Data encoding and type safety
  • [x] review ready
  • [ ] initial review
  • [ ] working example

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[laysakura]

@dahlbaek Could you review the (7) Data encoding and type safety when you have time?

[laysakura]

@dahlbaek (3) PCollections also got review ready!

[dahlbaek]

Of course, I'll have a look one of the next few days!

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