Document atomicity and concurrency guarantees

[gkorland]

Context

Migrated from FalkorDB/FalkorDB#227 (originally from RedisGraph).

What's Missing

The documentation has no section explaining the atomicity and concurrency model of FalkorDB. Users need to understand:

1. Single query atomicity — Is everything within a single query with any combination of CREATE/SET/DELETE/MERGE atomic? If a query fails mid-execution, are partial changes rolled back?
2. Compound clauses — Does atomicity cover write queries with UNION or WITH? Are multiple clauses executed as one atomic unit?
3. Write ordering — What is the ordering of CREATE/SET/DELETE/MERGE operations within a single query? Can the results of a CREATE be used in a subsequent SET within the same query?
4. Concurrent queries — How does FalkorDB handle concurrent read and write queries? What isolation level is provided? The thread pool (THREAD_COUNT) config implies parallelism — what guarantees exist?

Suggested Location

A new page under /design/ or /getting-started/ (e.g., concurrency.html or atomicity.html), linked from the main Cypher clauses page.

Why This Matters

Users building production applications need to understand whether they need external locking, transactions (MULTI/EXEC), or if single queries provide sufficient guarantees for their use case.