Removal of public devnet pages

Removal of public devnet of Babylon Gnesis due to low usage.

Devnet information are mentioned in following pages:

Simple removal of references:

- docs/guides/overview/bsns/cosmos_chains/cosmos_chains.mdx – dedicated section “Phase-3 devnet information” defining shell env vars for chain ID, RPC, GRPC, faucet. Remove it.

- docs/guides/overview/babylon_genesis/babylon_genesis.mdx – brief mention that certain behavior occurs “in devnet.” Remove the notice.

- docs/developers/babylon_genesis_chain/node_information.mdx – table of Edge devnet endpoints (RPC/LCD/GRPC) for Babylon Genesis. remove it.

- docs/developers/babylon_genesis_chain/chain_information.mdx – chain-information table listing edge-devnet-1, remove it.

- docs/operators/operators.mdx – operator overview bullet referencing “Phase 3 devnet (Multi-staking on BSNs).”, remote it.

- docs/operators/babylon_node/babylon_cli/validate_genesis.mdx – sample command pointing to ./devnet-genesis.json.

- docs/operators/babylon_node/babylon_cli/testnet.mdx – CLI flag example writing into ./devnet. Replace it with testnet information.

Following requires some checking and validation before switching endpoints and guides to testnet:

- docs/guides/overview/bsns/cosmos_chains/integrating_as_cosmos_chain.mdx – overall integration story for phase-3 Edge devnet, prerequisites, faucet instructions, CLI snippets pointing at Edge endpoints. Replace with testnet instructions, you will need to validate if testnet paramerter can work. If not make a contribution to the repo and confirm with original integrations team, Runchao, to see if the script can work with testnet tags.

- docs/developers/dapps/smart_contract_deployment.mdx – tells readers to source env-phase3-devnet.sh for multi-staking deployments. remove devnet script. Also go to source github.com remove the script for devnet, make a PR and tag me for approval.

- docs/guides/overview/bsns/cosmos_chains/running_finality_provider.mdx – version tags (v1.99.0-devnet.6), TODO about first devnet, RPC/GRPC endpoints hardcoded to rpc-edge.devnet.babylonlabs.io. remove this version from the versioning and replace it with testnet version.

- docs/developers/babylon_genesis_chain/explorers/explorers.mdx – Explorer link pointing at devnet.babylon.explorers.guru. Remove this, we will instruct node.guru to shutdown this service.

- docs/developers/staking_backend/services/staking_indexer.mdx – service configuration referencing rpc-dapp.devnet.babylonlabs.io. Replace it with testnet rpc: https://rpc-dapp.testnet.babylonlabs.io/.

- docs/guides/overview/bsns/op_stack_chains/op_stack_chains.mdx – links to Snapchain repo instructions for spinning up an OP Stack chain against Babylon Edge devnet.

- docs/operators/staker_cli/staker_cli.mdx – CLI config example for Babylon devnet (bbn-dev-5, RPC/GRPC hostnames). Reaplce with testnet GRPC and testnet chainID.