Document need to refresh DiscoURL

Document in [amppkg.example.toml](https://github.com/ampproject/amppackager/blob/6a1c7afdb77f4fb54a848e49deccdf5fa01ff76f/amppkg.example.toml#L216-L219) that the ACME discovery URL itself may expire and need to be refreshed. (Link to any CA documentation that's available on when it expires.)

Start with lines 216-219 of amppkg.example.toml and review the existing ACME discovery URL documentation. Find available CA guidance on when the discovery URL expires and add a link plus a note that it may need refreshing. Done means the example clearly documents both the expiration risk and the refresh requirement.

Written by the indexing model from the issue text.