Update README with explicit data-structure file mappings

[Nullkernel]

Motivation

• Make the Data-Structures section of README.md list each implementation with an exact one-to-one file path to improve discoverability and avoid ambiguous grouped bullets.
• Surface implementations that were present but not referenced (for example DoubleLinkedList and PriorityQueue).
• Move exploratory trie variants into a clearly labeled Additional/Optional Implementations subsection to indicate these are optional/extra.

Description

• Rewrote the Data-Structures list so each bullet references the concrete file path (for example, Data-Structures/Linked-Lists/DoubleLinkedList.java).
• Added missing entries including Data-Structures/Linked-Lists/DoubleLinkedList.java and Data-Structures/Queue/PriorityQueue.java to the main list.
• Added ### Additional/Optional Implementations and placed Data-Structures/Trees/Trie/Trie.java, PrefixTrie.java, and SuffixTrie.java there.
• Verified the README bullets map to existing repository files and kept the rest of the README unchanged.

Testing

• Ran git -C /workspace/DSA status --short and git -C /workspace/DSA diff -- README.md to validate the change was present and the diff matched expectations, which succeeded.
• Committed the updated README.md with git -C /workspace/DSA commit, which completed successfully.
• Inspected the final file contents with nl -ba /workspace/DSA/README.md to confirm the new section layout and entries, which succeeded.

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Codex Task