fix(docs): inconsistent table header in connecting-to-base

Path: /base-chain/quickstart/connecting-to-base

**Page:** https://docs.base.org/base-chain/quickstart/connecting-to-base
**File:** `docs/base-chain/quickstart/connecting-to-base.mdx`

## Description

The MetaMask manual setup table for Base Sepolia has an incorrect and inconsistent first row.

**Current (Base Sepolia table):**
| Name | Sepolia |
| Network Name | Base Sepolia |
| ... |

The header row says `Name | Sepolia` which is neither a proper column header nor a data row. The equivalent Base Mainnet table uses `Name | Value` as the header and includes a `Description` row with value `The public mainnet for Base.`

**Base Mainnet table for comparison:**
| Name | Value |
| Network Name | Base Mainnet |
| Description | The public mainnet for Base. |
| ... |

## Suggested fix

Fix the Base Sepolia table header to `Name | Value` and add a missing Description row:

```
| Name | Value |
| --- | --- |
| Network Name | Base Sepolia |
| Description | A public testnet for Base. |
| RPC Endpoint | https://sepolia.base.org |
...
```