Add basic navigation structure

[calvellido]

This PR includes some starting foundations for the site:

• It adds some basic styling such as colours, headings, adjust the element's global radius… And it also includes the Inter typography in a variable set, to be used along all the site. It also updates the site to show Kotlin code snippets properly highlighted (colours theme to be updated in the future).

• It makes the necessary changes in the navigation bar and the footer to include the proper links outlined in the site wireframes. It does so while also updating the pages placeholder template to make these pages use navbar and footer now.

• It configures and start using a method to use YAML (*.yml files) as a data source for pages. I think this go in line with the front matter data for Markdown files in the docs and blog pages, which might make it easier for non-technical people to make changes in case they deemed necessary. The code for this is also very clean, and it looks exactly the same way as we import styles for the pages, which makes it reasonable.

[nomisRev]

Looking good! Thanks @calvellido 🙌

[israelperezglez]

Good Job buddy. I like this variable ;)

--ifm-global-radius: 0;