Improve documentation for local docs build

[stes]

This PR makes it easier to locally build and test the documentation.

The components are:

• Update the contribution guide which will get published on https://cebra.ai/docs/contributing.html
• Add a script at ./tools/build_docs.sh which will check-out all additional repositories, trigger a docs build, and start a webserver to inspect the build at http://127.0.0.1:8080/

[MMathisLab]

(Just a simple permission issue it seems)

+ git clone git@github.com:AdaptiveMotorControlLab/cebra-assets.git assets
Cloning into 'assets'...
git@github.com: Permission denied (publickey).

[stes]

@clabot

[stes]

@clabot check