docs: add Infrahub Marketplace documentation

[minitriga]

Summary

• Adds a new topic page explaining what the Infrahub Marketplace is, how namespace/name identifiers work, and the relationship between the Marketplace and a local Infrahub instance
• Adds a how-to guide for fetching a schema with infrahubctl marketplace get and loading it with infrahubctl schema load
• Updates the getting-started tutorial and installation guide with Marketplace callouts
• Replaces all references to the deprecated GitHub schema library (github.com/opsmill/schema-library) in active docs with links to marketplace.infrahub.app

Relates to: infp-528 | Depends on SDK PR: opsmill/infrahub-sdk-python#952

Test plan

• Browse to /topics/marketplace and confirm the topic page renders correctly
• Browse to /guides/marketplace-install-schema and confirm all steps and commands are accurate
• Verify the getting-started tutorial and installation guide show the Marketplace callout
• Confirm no active docs link to github.com/opsmill/schema-library
• uv run invoke docs.lint passes with no new errors
• uv run invoke docs.build completes with no broken links

🤖 Generated with Claude Code

[BaptisteGi]

Let's put this one on hold until the end of the migration, I'll get it merged once the doc refactoring is done

[github-actions]

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