Updated CAN bus documentation and troubleshooting

[KevinOConnor]

This provides an assortment of documentation (and some diagnostic changes) for Klipper's CAN bus implementation. Notably:

1. This changes the default CAN bus frequency from 500Kbit to 1mbit in "make menuconfig".
2. I've removed references to the "waveshare rpi can hat", as that adapter has always been a bit finicky. The USB CAN adapters are now relatively easy to get, so I've updated the documentation to indicate they are preferred.
3. There is a new CANBUS_Troubleshooting.md document with tips on diagnosing Klipper canbus communication failures.
4. I've added additional information on "USB to CAN bridge" limitations.
5. I've added a new parsecandump.py diagnostic tool (this PR replaces #6176).

-Kevin