docs(logger): add doxygen documentation comments

[kshku]

Add Doxygen-style documentation comments to headers.

Files to document

• logger.h
• logger.c

Format to follow

Use the same style as the Sn* libraries already used in this project:

/**

• @brief One line summary.
• multi-line description.
• @PARAM name Description.
• @return Description.
• @note Any ownership, lifetime, or thread-safety requirements.
  */

What counts as done

Every public function, struct, enum, etc., in the header has a Doxygen doc comment.
Internal static functions in the corresponding .c file should have at minimum a @brief comment explaining what they do and any non-obvious behavior, ownership, or assumptions.

Good to know

• Read through docs/snuk_language.snuk for context on what the language does
• Look at external/SnLogger or external/SnMemory headers for examples of the documentation style already used in this project