Launch a Token guide uses raw PRIVATE_KEY env variable, contradicting Deploy Smart Contracts guide's safer keystore approach

## Description
The "Launch a Token" guide uses `vm.envUint("PRIVATE_KEY")` to load a
private key directly from a `.env` file in the deployment script.

However, the "Deploy Smart Contracts" guide on the same site explicitly
teaches `cast wallet import deployer --interactive` as the secure approach,
with a warning: "Never share or commit your private key."

These two pages give contradictory advice on a security-critical topic
with no explanation of the difference.

## Pages affected
- https://docs.base.org/get-started/launch-token (uses raw env var)
- https://docs.base.org/get-started/deploy-smart-contracts (uses keystore)

## Suggested fix
Either align both guides to use the keystore method, or add a note on the
Launch a Token page explaining when a raw PRIVATE_KEY env var is acceptable
(e.g. only in CI/CD pipelines, never locally) and link to the safer approach.