Add documentation for Mach-O and dex modules

[wesinator]

Mach-O and dex (#822) modules are missing from https://yara.readthedocs.io/en/latest/modules.html

[metthal]

I've recently ran into this too. I see that there's a PR for mach-o already. My knowledge of DEX is rather limited. I don't know if original author of DEX module (https://github.com/adesnos - can't tag him, he used different account to submit PR which is no longer active) is interested in it.