docs: add SolidRPC node provider

[DamianFigiel]

What changed? Why?

Added SolidRPC to the Base node provider directory.

SolidRPC supports Base Mainnet and Base Sepolia through authenticated JSON-RPC endpoints over HTTPS. The service provides full and archive access, standard Ethereum methods, and trace/debug APIs for Base. A free plan is available, with paid plans for higher limits and production workloads.

• Website: https://solidrpc.io
• Documentation: https://solidrpc.io/docs
• Supported networks: https://solidrpc.io/docs/networks

Notes to reviewers

This change only adds a provider entry and follows the structure used by the existing node-provider listings. It does not modify Base network configuration or recommend a keyless public endpoint.

How has it been tested?

Documentation-only change.

• Verified Base Mainnet and Base Sepolia against the current SolidRPC network catalog
• Ran git diff --check

[cb-heimdall]

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