Shorten Contributing Guidelines by introducing subsections that can be collapsed

[likeajumprope]

Is your feature request related to a problem? Please describe.
The Contributing Guidelines are very long.

Describe the solution you'd like
Divide the contributing guidelines into subsections that can be collapsed., as seen here: https://github.com/htwangtw/depersonalisation

Describe alternatives you've considered
Alternatively, we could improve the structure of the Contributing Guidelines.

Additional context
For an example of how the output should look like, see here:
https://github.com/htwangtw/depersonalisation

[jhlegarreta]

Particularly, I find the git/repository cloning related instructions (e.g. how to set the upstream remote, pulling changes) something that, even if useful, is required in virtually any git-based project. So I would shorten those passages.

Same applies to the paragraphs about creating a virtual environment: we could just mention that a virtual env is the preferred options to install the dependencies: e.g.

(...)
1. If you are using `venv` or `virtualenv` create a virtual environment. Once created and activated, do a
pip install -r requirements.txt
2. Conda-related option (...)

and put the links to the docs of venv or virtualenv.

Also, I think the sections that come after Bibliography could be put somewhere else, since they are not strictly related to the guidelines to install the requirements or build the book, but are rather about features of a book/jupyter notebook.

[htwangtw]

I am in favour of a collapsable section over simplifying the instructions with link to the official doc. After interacting with some people at Brainhack, the new people who approached me mostly need detailed guidance on set-up. It would be useful to have all info in one place to point people to.

Talking about set-up, we still need some help to have some instruction working for windows, see #74

[likeajumprope]

will have a look