docs(base-account): missing documentation for Session Keys (critical feature)

## Problem

Base Account supports **Session Keys** (also known as delegated signers or spending keys) to allow AI agents, games, and automated scripts to perform transactions on behalf of the user without requiring manual approval for every action.

However, there is **no documentation** explaining how to use this feature:
- No guide in `/docs/base-account/guides/`
- No API reference in `/docs/base-account/reference/`
- No RPC method documentation for session key management (e.g., `wallet_grantPermissions`, `wallet_revokePermissions`)

Developers cannot implement this core smart wallet feature because they don't know:
1. How to create a session key with specific permissions (e.g., only USDC transfers, max $10/day).
2. How to set an expiry time for the key.
3. How to revoke a session key.
4. Which SDK methods or RPC calls to use.

## Expected Behavior

Add comprehensive documentation for Session Keys, including:

### 1. A New Guide: `docs/base-account/guides/session-keys.mdx`
- Explain what Session Keys are and their use cases (AI agents, gaming, subscriptions).
- Step-by-step tutorial on creating a session key using `@base-org/account` SDK.
- Code examples showing:
- Setting permissions (allowed contracts, methods, value limits).
- Setting expiry time.
- Revoking the key.
- Security best practices (short expiry times, least privilege principle).

### 2. API Reference Updates
- If Session Keys are managed via RPC methods (e.g., `wallet_grantPermissions`), add documentation in `/docs/base-account/reference/core/provider-rpc-methods/`.
- If managed via SDK methods, document them in `/docs/base-account/reference/core/`.

## Why This Matters

Session Keys are a key differentiator for Smart Wallets like Base Account. Without documentation, developers cannot build seamless UX experiences (e.g., gasless, signature-less interactions) that Base promotes.

## References

- Base Account Overview: https://docs.base.org/base-account/overview
- EIP-5792 (Wallet Capabilities): https://eips.ethereum.org/EIPS/eip-5792