docs: reorganize learn foundations

[frydj]

Summary

Restructures Learn > Foundations section:

Added

• what-makes-fluree-different.mdx: Fluree's unique value proposition
• how-fluree-stores-data.mdx: Immutable ledger and triple storage
• understanding-graph-relationships.mdx: Nodes, edges, and traversals
• understanding-context.mdx: JSON-LD context basics
• index.md: Learn section overview

Removed

• data-access-control.mdx (moved to security section)
• querying.mdx (replaced by working-with-data guides)
• overview.md (replaced by index.md)

Modified

• json-ld.mdx: Minor updates for consistency

Test plan

• Review new content for accuracy
• Verify removed content is covered elsewhere in stack

[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 👈 (View in Graphite)
• docs: add getting started guides #60
• docs: add project config & infrastructure #59
• main

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