Add recommended next steps for How to build a Linux kernel

The current [How to build an Ubuntu Linux kernel](https://canonical-kernel-docs.readthedocs-hosted.com/en/latest/how-to/develop-customise/build-kernel/) guide is complete.

But as the SMEs it would be good to recommend what the user could do next.

E.g. use `cranky` (or any other recommended method) to build the customised kernels in Launchpad since locally built packages are difficult to deploy. Whereas moving to Launchpad PPA builds would mean proper environment setup, make it easier to deploy the kernel source packages, infrastructure is ready for building kernels across different architectures as needed.