Gonzalo provided a good overview of the different modules.
Since you also mentioned wanting to contribute to documentation, you'll want a few more pieces of information.
First, Documentation is stored in XML files within mcs/class/[assembly]/Documentation/, e.g. mcs/class/corlib/Documentation. The intent is to support multiple human languages (though only English is currently being worked on), so within Documentation is a language directory, usually en. Within en there are ns-*.xml files, e.g. mcs/class/corlib/Documentation/en/ns-System.xml contains documentation for the System namespace. Also within en are "dotted namespace" directories, and within those are XML files, one per type, for example mcs/class/corlib/Documentation/en/System.Collections.Generic/IEnumerable`1.xml.
This is also outlined within the mdoc(5) documentation, in the FILE/DIRECTORY STRUCTURE section.
Once you've found the documentation, you need to know the XML format, which is also described in the mdoc(5) documentation, in the NamespaceName/TypeName.xml File Format section. The XML dialect used is a variant of the ECMA 335 XML documentation, changed to have one file per type (instead of all types within a single monolithic file). This is also a superset of C# XML documentation (see Annex E. Documentation Comments, page 487).
Finally, there's the question of adding new types/members to the mcs/class/[assembly]/Documentation directory. If you have Mono built, you can use the doc-update Makefile target. This will run the appropriate assembly through mdoc(1) and update the appropriate files within the Documentation directory.
If you have any other documentation questions, don't hesitate to ask on the mono-docs-list mailing list.
