Docs: Add documentation on restart usage and logic

[mbercx]

Understanding and implementing all the restarting use cases in Quantum ESPRESSO is not for the faint of heart. We've done a lot of work lately on improving the restart functionality in aiida-quantumespresso, and the number of discussions on the topic spread of various issues and PRs is massive. Now we should do the exercise of writing all this knowledge down in the documentation. Specifically, we should:

• Give a basic explanation of how restarting works in QE (at least for pw.x).
• Document validation and other implementation choices in some "design" documentation.
• Document the usage of restarting calculations in how-to's.

Some related issues and PRs that contain valuable discussions:

• PwBaseWorkChain: revisit restarting from calculations #691
• PwBaseWorkChain: improve restart from parent_folder #722
• Protocols: Move all static work chain inputs to protocol #902
• PwCalculation: Fix restart validation for nscf/bands #906