Third tutorial

[louilinn]

PR fixes https://github.com/CirclesUBI/circles-handbook/issues/9 and fixes https://github.com/CirclesUBI/circles-handbook/issues/8

• [x] Reformat tutorial to markdown
• [x] Add tutorial(s) to sidebar
• [x] Collapse extensive code blocks
• [ ] Consider other changes to the tutorial in integrating it to the docusaurus website
• [x] Make images smaller / better in first two tutorials

[louilinn]

I changed the images in the tutorials as well, looked for different options but ended up using this: https://docusaurus.io/docs/markdown-features/react

Not sure if I should put that code for FormattedImage somewhere else if we decide to stick with this as it is now duplicated.

[adzialocha]

Not sure if I should put that code for FormattedImage somewhere else if we decide to stick with this as it is now duplicated.

Let's move it to /src/common/FormattedImage.js!