[doit] add task Documentation

[umarcor]

Similarly to #162, this is a subset of #110.

Instead of calling make in the Documentation workflow, a task named Documentation is added to the do.py file.
For now, the following are equivalent:

• ./do.py Documentation
• doit -f do.py Documentation
• make -C docs

In upcoming PRs, we might remove docs/Makefile and replace it with more specific doit tasks.

[stnolting]

Awesome! 👍

In upcoming PRs, we might remove docs/Makefile and replace it with more specific doit tasks.

Let's keep that for now.