docs: restructure querying reference

[frydj]

Summary

Restructures Reference > Querying > FlureeQL from single file to organized directory:

• index.mdx: Query structure overview
• select-clauses.mdx: Select syntax and patterns
• where-clauses.mdx: (refactored from flureeql-query-syntax.mdx) Where clause patterns
• aggregations.mdx: count, sum, avg, etc.
• modifiers.mdx: limit, offset, orderBy, groupBy
• advanced-features.mdx: Subqueries, unions, optional patterns

Removed

• flureeql-query-syntax.mdx: Content split into directory structure

Test plan

• Verify query examples return expected results
• Check all syntax variations are documented

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

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