Write intro chapter for documentation / tests for all functionalities

[dbosk]

The chapter should decribe how to use the makefiles. A good way is to literally
document the Makefile used in the repo. This Makefile uses a lot of the
available functionality. Thus it can also serve as "tests" for the functionalities.

[dbosk]

The makefile for texgen is a good example for tex.mk, noweb.mk and package.mk.

[dbosk]

The intro chapter should test all functionality, thus we can merge #3 with this issue.

[dbosk]

The intro chapter now documents the repo Makefile. This illustrates some use, but not all.

[dbosk]

This is related to #69.