Add script to filter Doxygen warnings

Add a list of expected warnings from Doxygen, a Python script to
filter out those warnings from the output from Doxygen, and a simple
bash script to run doxygen using this filter.

In some cases, it would be relatively easy to fix the Doxygen
documentation so that we don't get these warnings.  However, in other
cases there are bugs in Doxygen where the code and documentation are
perfectly valid, and Doxygen gives an erroneous warning message.

Currently, CI uses another script to filter out warnings related to
lines of code which have not been changed.  However, this doesn't work
well because the line number reported for a warning is often not the
line where the problem lies.

The new script should be useful for CI, and also for checking changes
locally before running CI.

Author: John Nonweiler

doc/architectural/compilation-and-development.md

@@ -194,8 +194,10 @@ tools. All of these could be improved and patches are very welcome. In
 some cases the algorithms used are described in the relevant papers.
 
 The doxygen documentation can be [accessed online](http://cprover.diffblue.com).
-To build it locally, run `doxygen` in `src/`. HTML output will be created in
-`doc/html/`. The index page is `doc/html/index.html`.
+To build it locally, run `scripts/run_doxygen.sh`. (This script will filter out
+expected warning messages from doxygen, so that new problems are more obvious.)
+HTML output will be created in `doc/html/`. The index page is
+`doc/html/index.html`.
 
 
 \section compilation-and-development-section-formatting Formatting