Linking installation and structure of README.md with installation.rst and structure.rst

[MaGering]

With the pages on installation and structure we have redundant information in the README.md and the docs. This should be streamlined by keeping only one place and including this into both. But we can tackle this later on.

Originally posted by @nesnoj in #7 (review)

[MaGering]

I wanted to solve this by importing the respective section from README.md in the rst-file like this, for installation:

.. include:: ../../README.md
   :literal:
   :start-after: Installation
   :end-before: Further reading on structure, pipeline and conventions

However the options start-after and end-before are not implemented yet in m2r2 (see CrossNox/m2r2#31)

[MaGering]

A much more troublesome workaround would be via the options start-line and end-line:

.. mdinclude:: ../../README.md
   :start-line: 12
   :end-line: 47

But here you have to specify the line numbers, which can change as soon as the README.md is changed. Therefore this option is maintenance-intensive or is very error-prone if it is forgotten.

[nesnoj]

Rediscuss as soon as we have decided for or against mkdocs

[nesnoj]

Waits for #88

[nesnoj]

Outdated as mkdocs were implemented in #88