Improving the look of the start and example page in docs

[nahakiole]

I've added some styling from the docs to the start and example page so it looks and feels like it's the same website.

You can see the changes here: https://nahakiole.github.io/vis-network/ and here https://nahakiole.github.io/vis-network/examples/ on my fork.

In the contribution guide it's written that the pull request should be made to the gh-pages branch, but here in the checklist of the pull request it's only the master branch?

https://github.com/visjs/vis-network/blob/master/misc/how_to_help.md

The visjs.org webpage is hosted on the gh-pages branch. If you find a typo or anything else that should be improved feel free to create a pull-request to gh-pages. Please make changes in your own fork of gh-pages so the support team can view the changes in your hosted fork.

All pull requests must be to the master branch. Pull requests to any other branch will be closed!

If I need to move the commit to the other branch please tell me, and then I'll add a commit to update the how_to_help.md as well.