feat: Add Doxygen documentation and Gitea workflow

This commit introduces Doxygen for documentation generation and a Gitea
workflow for continuous integration.

- Adds a `Doxyfile` to configure Doxygen to build a documentation
  website with diagrams.
- Updates `AGENTS.md` to require Doxygen-compatible comments for all
  code.
- Adds Doxygen comments to existing C++ source files to comply with
  the new standard.
- Creates a Gitea workflow (`.gitea/workflows/build.yml`) that:
  - Builds the project on every push.
  - Generates Doxygen documentation.
  - Uploads the compiled library and the documentation website as
    artifacts.

Doxyfile

@@ -0,0 +1,49 @@
+# Doxyfile 1.9.1
+
+# Project
+PROJECT_NAME           = "MedicalLib"
+PROJECT_NUMBER         = 1.0
+PROJECT_BRIEF          = "A C++ library for medical simulations"
+OUTPUT_DIRECTORY       = "docs"
+CREATE_SUBDIRS         = YES
+OUTPUT_LANGUAGE        = English
+BRIEF_MEMBER_DESC      = YES
+REPEAT_BRIEF           = YES
+
+# Sources
+INPUT                  = ./include ./src
+FILE_PATTERNS          = *.h *.cpp
+RECURSIVE              = YES
+EXCLUDE_PATTERNS       =
+EXCLUDE_SYMBOLS        =
+
+# HTML
+GENERATE_HTML          = YES
+HTML_OUTPUT            = html
+HTML_FILE_EXTENSION    = .html
+HTML_HEADER            =
+HTML_FOOTER            =
+HTML_STYLESHEET        =
+HTML_COLORSTYLE_HUE    = 220
+HTML_COLORSTYLE_SAT    = 100
+HTML_COLORSTYLE_GAMMA  = 80
+
+# Diagrams
+HAVE_DOT               = YES
+DOT_TOOL_PATH          =
+DOTFILE_DIRS           =
+UML_LOOK               = YES
+UML_LIMIT_NUM_FIELDS   = 10
+TEMPLATE_RELATIONS     = YES
+CALL_GRAPH             = YES
+CALLER_GRAPH           = YES
+GRAPHICAL_HIERARCHY    = YES
+DIRECTORY_GRAPH        = YES
+DOT_IMAGE_FORMAT       = png
+INTERACTIVE_SVG        = NO
+DOT_GRAPH_MAX_NODES    = 50
+MAX_DOT_GRAPH_DEPTH    = 0
+DOT_TRANSPARENT        = NO
+DOT_MULTI_TARGETS      = YES
+GENERATE_LEGEND        = YES
+DOT_CLEANUP            = YES