Add initial project files for flow meter design

- Created .giignore to exclude Altium Designer project output and temporary files.
- Added baseboard schematic document (baseboard.SchDoc).
- Introduced flow meter project PCB file (flow_meter.PrjPcb) with detailed configuration settings.
- Defined project structure in flow_meter.PrjPcbStructure linking to the baseboard schematic.
- Added top layer PCB document (topLayer.PcbDoc) for the flow meter design.

.giignore

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+# Altium Designer Project Files
+
+# Project Output
+*.PcbDoc.htm
+*.SchDoc.htm
+
+# History and Backup Files
+History/
+__Backups/
+__Previews/
+*.~(pcbdoc|schdoc|outjob|pcb|sch)
+
+# Output Files
+Project Outputs for*/
+*.PDF
+*.DWF
+*.DXF
+*.ODB
+*.XLS
+*.CSV
+
+# Gerber Files
+*.GBR
+*.GBO
+*.GBP
+*.GBS
+*.GKO
+*.GKP
+*.GTL
+*.GTO
+*.GTP
+*.GTS
+
+# NC Drill Files
+*.NC
+*.XNC
+*.TXT
+
+# Assembly Files
+*.Txt
+
+# CAM Files
+*.CAM
+
+# Libraries
+*.IntLib
+*.PcbLib
+*.SchLib
+*.LibPkg
+
+# Temporary Files
+*.tmp
+~*.tmp
+
+# State Files
+*.dpk
+State/
+
+# OS Files
+.DS_Store
+Thumbs.db
+
+# IDE
+.vs/
+.vscode/
+
+# Build Artifacts
+bin/
+obj/

flow_meter.PrjPcbStructure

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+Record=TopLevelDocument|FileName=baseboard.SchDoc|SheetNumber=1