docs: add getting started guides

[frydj]

Summary

New Getting Started section with 4 guides:

• index.md: Section overview and navigation
• quick-start.mdx: 5-minute intro to creating a ledger and running queries
• installation.mdx: Docker, JAR, and Cloud setup instructions
• core-concepts.mdx: Key concepts (ledgers, JSON-LD, triples, immutability)

Test plan

• Review guides for accuracy and completeness
• Verify code examples are copy-paste ready

[frydj]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• docs: add feature showcase examples #71
• docs: add domain example datasets #70
• docs: add feature reference docs #69
• docs: reorganize cookbook #68
• docs: restructure querying reference #67
• docs: restructure HTTP API reference #66
• docs: add advanced topics #65
• docs: add security guides #64
• docs: add modeling guides #63
• docs: add working with data guides #62
• docs: reorganize learn foundations #61
• docs: add getting started guides #60 👈 (View in Graphite)
• docs: add project config & infrastructure #59
• main

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