adjust link and explanation code styleguide

[PrometheusPi]

This pull request solves #4818

It adds the link to the style guide and explains that the repo linked before is the discussion on the code style.

• check if links in *.md work as in *.rst

[chillenzer]

Thanks for taking care of this! To my own surprise the [name](link) syntax seems to work. But the :ref:... syntax failed.

[chillenzer]

Content-wise this seems okay. It's a very minimal solution and does not fully resolve #4818. That was more targeted towards a general overhaul of also docs/source/dev/styleguide.rst because that's outdated, too. But don't let the perfect be the enemy of the good, so please go ahead with this one but leave the issue open.

[PrometheusPi]

The linking works differently in *.md then in *.rst - this the link is not shown properly.
Please do not merge.

[PrometheusPi]

Linking runs into the following issue #4831

[chillenzer]

Which is probably precisely the reason why this was not done properly last time. XD Feel free to close #4831 first!