Change repo layout for github.io

[teodutu]

We migrated all of our tracks to github.io [1]. This requires some modifications to the structure of each repository:

• Rename session documentation files from README.md to index.md
• Rename the Markdown file at the root of the repository from README.md to _index.md
• Add a Github action that updates the content on github.io (where each repository is imported as a submodule). This step may be left as a stub / not functional in order not to have Github action failures upon each merged PR.

[1] https://github.com/security-summer-school/security-summer-school.github.io

[gabrielmocanu]

Done.