Update Canton Docker image docs

[danielporterda]

Closes #526.

Summary

• replace the old Docker Hub image link in the Canton getting-started tutorial with an internal link to the maintained Docker image instructions
• clarify that the Docker pull examples use an example version tag and readers should substitute the Canton version they want

Validation

• direnv exec . git diff --check
• direnv exec .. npx mintlify validate from docs-main
• local Mintlify preview on http://localhost:3026

Screenshots

Getting started installation now links to the Docker image instructions instead of Docker Hub.

Docker image instructions now tell readers to replace the example 3.4.8 tag with the version they want.

[mintlify]

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Project	Status	Preview	Updated (UTC)
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