Write a tutorial about how to work on the site locally

[matks]

Write a tutorial about how to work on the site locally #1103

Information on how to run the site locally should be updated here

This is tricky because it involves using submodules. @eternoendless does this: cloning devdocs-site, then working on the submodule directory and then push to own docs remote

Items completed

1. [x] The docs is about how to contribute. Reference information on how the devdocs is deployed should go to the README in https://github.com/PrestaShop/devdocs-site/ => pull requests

• https://github.com/PrestaShop/devdocs-site/pull/1
• https://github.com/PrestaShop/docs/pull/1104

2. [x] Branch information should be added as a "Documentation versions" subsection here. I think it would be best to add it at the top, before "Edit on Github". => pull request:

• https://github.com/PrestaShop/docs/pull/1106