Docusaurus installation ready for content

Hi,

as discussed with @gtsouk, I created a docusaurus installation in essif-lab, which will serve the purpose for the documentation site for essif lab.

Right now it's a simple setup, all you need to run this is yarn. Information are on the README.md. I also added a commit on how to add a new page in docusaurus, along with declaring it on the sidebar, and on the menu bar. You can see it here.

@fmerg as @gtsouk mentioned, you can now start adding content in this project.

Next steps for this project:

• Add a dev Dockerfile for easier development
• Add a gitlab-ci.yml job to automatically build this project and deploy it in pages
• Theme it to a more appropriate style