Docs: How-To: Write custom rules

[chr5tphr]

Add a How-To on writing custom rules, specifically using

• zennit.core.Hook for general Hooks, and
• zennit.core.BasicHook for LRP-based Hooks

For zennit.core.Hook:

• add explanations for backward and forward and zennit.core.Hook
• give a short formal demonstration for the parts of the gradient in the
  backward function of Hook
• introduce copying mechanics and Hooks with arguments

For zennit.core.BasicHook:

• give a short description of the different functions used by BasicHook
• give a rough formal definition of the computation done by BasicHook
  using the different functions
• give a few examples, with a simple instance, and subclassing of
  BasicHook
• explain the default values and mechanics for the BasicHook argument
  functions