embed doxygen link in CONTRIBUTING.md

[2bndy5]

starting to get doxygen now. But the official reference manual seems a little vague. e.g. I can't find where the "@" (like @param) commands are explained/listed.

added link to "documenting the code section" to steer prospective contributors in the official direction.

[2bndy5]

nevermind. I think I found it:

All commands in the documentation start with a backslash () or an at-sign (@). If you prefer you can replace all commands starting with a backslash below by their counterparts that start with an at-sign.

I knew I'd find it eventually.