New: gantry mgn9h parts

[heartpolik]

• [x] The mod, firmware or slicer profile is in the correct category folder. Printable mods go to Mercury One/, slicer profiles go to slicer_profiles/. Create a subfolder with your name, and place the mods in a subfolder with a descriptive name within that folder, e.g.: /Mercury_One/FHeilmann/flux_capacitor
• [x] Folders names MUST NOT contain spaces. If possible, make sure file names also do not contain any spaces.
• [x] For each mod, add a small README.md file to its folder with a short description of what the mod accomplishes. This readme can be used to add pictures, give assembly instructions or specify a bill of materials if the mod requires additional hardware.
• [x] The PR modifies the top-level README.md of the category folder adding the contribution to the table. Read the top part of the file for instructions on how to do this. Please preserve the alphabetical ordering while adding new rows.
• [x] The mod/profile has been tested by the person submitting the mod and/or other ZeroG users. Make sure to add information about how the mod was tested below.
• [x] The mod is not merely a slight modification of an official ZeroG part profile (i.e. an official ZeroG part with a few mm added or removed or a slicer profile which only modifies a few values). (When in doubt, contact one of the admins in the ZeroG discord before submitting the PR)
• [x] Submitted STLs are printable without support. (If the mod does not meet this criterion join the ZeroG discord and ask the other users for advice on how to modify the mod such that it does not require supports)
• [x] Submitted STL files are not corrupt. (This can be tested by opening the STL in PrusaSlicer or SuperSlicer and checking if mesh errors are reported.)
• [x] Submitted STL files are oriented and scaled properly for printing.
• [x] Submitted slicer profiles contain no sensitive data (e.g. API keys).

Which mods/profiles are added by this PR?

How was it tested?

Any background context you want to provide?

Screenshots (if appropriate)

Further notes