[QA] nav: promote Networks to a top-level tab

[theekrystallee]

Which tab?

Learn → Networks (new)

Page URL or path

https://docs.hedera.com/learn/release-notes/

What's the problem?

Currently, Release Notes lives as a group under the Learn tab alongside Networks and getting-started content

Suggested fix (optional)

Promote Release Notes to its own top-level tab.

Learn              (conceptual only — Networks removed)
Networks           (new top-level tab)
   ├── Networks    (overview)
   ├── Mainnet     (access, fees, transaction records)
   ├── Testnet     (access, consensus nodes)
   ├── Localnet    (single-node, multinode)
   ├── Community Mirror Nodes
   └── Release Notes
          ├── Hedera Services
          └── Mirror Node