Fix documentation links

1) Make sure .c source files are considered by Doxygen. (Notably,
cprover_contracts.c contains relevant documentation.)
2) Reference other files in ways understood by Doxygen.

Author: tautschnig

doc/architectural/symex-instructions.md

@@ -23,12 +23,12 @@ Symex is, at its core, a GOTO-program interpreter that uses symbolic values inst
 This produces a formula which describes all possible outputs rather than a single output value.
 While Symex is interpreting the program, it also builds a list of Static Single Assignment (SSA)
 steps that form part of the equation that is to be sent to the solver. For more information see
-[src/goto-symex](../../src/goto-symex/README.md).
+\ref symbolic-execution.
 
 You can see the main instruction dispatcher (what corresponds to the main interpreter
 loop) at `goto_symext::execute_next_instruction`.
 
-Symex's source code is available under [src/goto-symex](../../src/goto-symex/).
+Symex's source code is available under \ref goto-symex.
 
 ## Instruction Types
 
@@ -115,7 +115,7 @@ case ASSUME:
   break;
 ```
 
-The way the [`symex` subfolder](../../src/goto-symex/) is structured, the different
+The way the \ref goto-symex subfolder is structured, the different
 dispatching functions are usually in their own file, designated by the instruction's
 name. As an example, you can find the code for the function goto_symext::symex_goto
 in [symex_goto.cpp](../../src/goto-symex/symex_goto.cpp)

doc/doxygen-root/doxyfile

@@ -853,6 +853,7 @@ INPUT_ENCODING         = UTF-8
 FILE_PATTERNS          = *.cpp
 FILE_PATTERNS         += *.h
 FILE_PATTERNS         += *.md
+FILE_PATTERNS         += *.c
 
 # The RECURSIVE tag can be used to specify whether or not subdirectories should
 # be searched for input files as well.