idler_bearing_cover not modded? / Add descriptions for Galileo2 step files

The printable files for the Galileo 2 extruder appear to fit the USB board well.

The idler_bearing_cover appears identical to the original in the Galileo 2 repo. Overlaying them in a slicer, I was unable to see any physical difference. Please clarify?

Also, the changes are very subtle. I found it confusing to figure out the new printable files and confirm after printing that I had the correct parts. It would be helpful to include some notes in a design README.md to clarify the differences and how to spot the them.

1. For the body and the cover, there are some small sections of material removed to make room for the board. It's easy to tell the modified parts from the old ones because the modified parts have a rounded corner near the hole at the top where the filament enters.
2. The new chain anchor removes the holder for the chamber thermistor.
3. I haven't investigated the usage of the cable clamp, so notes here would also be helpful.

Thanks for providing the mod that enables using the SB2209 USB with Galileo 2.

Start with the Galileo 2 printable files and the design README.md, comparing idler_bearing_cover with the original Galileo 2 version. Check the body, cover, chain anchor, and cable clamp changes, then document how to identify each modified part and clarify the bearing-cover difference and cable-clamp usage.

Written by the indexing model from the issue text.