New: gantry mgn9h parts

[heartpolik]

• 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
• Folders names MUST NOT contain spaces. If possible, make sure file names also
  do not contain any spaces.
• 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.
• 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.
• 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.
• 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)
• 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)
• 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.)
• Submitted STL files are oriented and scaled properly for printing.
• 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