Include a link to Doxygen documentation on the web

The Doxygen documentation is on
https://cclark/geeqie/doxygen/html/index.html

For there to be a search capability in the documentation the web server
must support php.
The github web site does not.

There may be a more suitable solution, but at the moment this is not
significant.

CODING.md

@@ -347,6 +347,8 @@ See the shell scripts section in the Doxygen documentation (`File List`, `detail
 
 ## Documentation
 
+The documentation in Doxygen format for the latest release, or later, is [here](https://cclark.uk/geeqie/doxygen/html/index.html).
+
 Use American, rather than British English, spelling. This will facilitate consistent
 text searches. User text may be translated via the en_GB.po file.