feat: Adding a tutorial for deploying UPF in DPDK mode

[gatici]

Description

The implementation of the DPDK support in the UPF has been introduced in https://github.com/canonical/sdcore-upf-operator/pull/57. However, documentation of this feature was missing. Hence, this PR adds a tutorial about using UPF in DPDK mode.

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have made corresponding changes to the documentation
• [ ] I have added tests that validate the behaviour of the software
• [ ] I validated that new and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [ ] I have bumped the version of the library