Adding a banner to docs about templating changes in core-2.19

[felixfontein]

@samccann

How about changing the last sentence to
Use <pointer to porting guide> to understand where you may need to update your playbooks and roles.

Sounds good to me!

@samccann

Then we could potentially add a sentence to the intro to the porting guide to say We recommend you test your playbooks and roles in a staging environment with stable-2.19 to determine where you may need to make changes.