Modifications in the documentation, PLL refactoring

[mhinkkan]

• Design notes are written for the phase-locked loop (PLL) in a similar manner as in the drives documentation. In order to provide links to grid-forming methods, the observer-based PLL approach is used. The documentation can be further updated later, including more detailed links to observer-based power synchronization.
• In the code, the low-pass filtering of the measured PCC voltage is moved to the PLL.
• Various minor refactoring and polishing.