template for gh pages landing page in /docs on master

[mihai-sysbio]

The template could facilitate having a GH Pages website. I think it doesn't need to be bound to the docs folder on main anymore. Below is a draft:

- [x] `/docs/ `
The repository can contain a `/docs/` folder. A default is provided by the `standard-GEM` template. Its purpose is to instruct GitHub Pages to create a webpage from the main `/README.md` in the repository. This webpage serves as a friendly landing page that requires no maintenance, since it will always be in sync with the `/README.md`.  
The configuration of this webpage is specified by the `_config.yml` file, and can be modified to the desire of the repository owners. As specified in this file, the icon at `/docs/icon.png` will be used on this webpage.
One need to instruct GitHub to create a website by the following steps:
1. navigate to the repository settings
2. scroll down to the `GitHub Pages` section
3. select as `Source` the `master branch /docs folder`
4. the website is now available at `http(s)://{{organization or user}}.github.io/{{repository name}}`, for example `https://sysbiochalmers.github.io/yeast-GEM/`

_config.yml:

theme: jekyll-theme-minimal
logo: docs/icon.png
show_downloads: "true"