Add project scaffold: pyproject, Doxyfile and package layout

MIL-001 Project scaffold.

- Add pyproject.toml: Python 3.13 or newer, no runtime dependencies,

  pytest as the dev extra

- Add src/calc/ package and tests/ folder

- Add Doxyfile writing API docs to docs/doxygen/

- Ignore docs/doxygen/ in .gitignore

Closes #15

Closes #16

Closes #17

Closes #18

Task: MIL-001#1

Task: MIL-001#2

Task: MIL-001#3

Task: MIL-001#4

Doxyfile

@@ -0,0 +1,16 @@
+# Doxygen configuration for the console calculator.
+# Generate the API documentation with:  doxygen Doxyfile
+PROJECT_NAME           = "Calculator"
+PROJECT_BRIEF          = "Console calculator (Udemy 100 Days of Code, day 10)"
+OUTPUT_DIRECTORY       = docs/doxygen
+INPUT                  = src
+FILE_PATTERNS          = *.py
+RECURSIVE              = YES
+OPTIMIZE_OUTPUT_JAVA   = YES
+EXTRACT_ALL            = YES
+EXTRACT_PRIVATE        = NO
+GENERATE_HTML          = YES
+GENERATE_LATEX         = NO
+QUIET                  = YES
+WARN_IF_UNDOCUMENTED   = YES
+WARN_NO_PARAMDOC       = YES