Use reference style Markdown links

[pkeilbach]

With 1b68990d124350a5b417041bc4a89818e7bb8013, I switched to reference style links for the README.md and index.md.

This makes the actual markdown file more readable and reduces line length issues.

I want to implement this for all markdown files in the project.

Each streamlined markdown file should be a separate PR and will be rewarded with 1 bonus point 🏅

For every streamlined markdown file:

• open an issue that states which markdown file you are going to streamline
• streamline all links in the file
• open a PR

Additional notes:

• Add the reference links on top of the page using a comment to indicate <!-- page links -->, so that developers easily spot already available links.
• Use simple reference links when possible, otherwise labeled reference links, e.g.:

<!-- page links -->
[Python]: https://docs.python.org/3/
[git]: https://git-scm.com/

# Hello World!

The code for this course is written in [Python].

We use [a very popular version control system][git] for this course.

Find more information in the markdown reference.

[MichalT106]

Hi can you assign me? I would like to work on this issue.

[pkeilbach]

@MichalT106 you will need to create a new issue which makes clear which file you are going to streamline ✌🏼 You can assign yourself directly to this issue.

For every streamlined markdown file:

• open an issue that states which markdown file you are going to streamline

this issue is just a tracking issue to make clear that this needs to be done for several files.