New style redesign

[AntonioMateoGomez]

This PR implements the new Sbt-microsites design wich we have named light. We also keep the classic pattern style and we provide the option to generate it aswell.
Documentation has change to describe how to set any of these two styles.
We remove bootstrap from the new style, also we change the default highlight theme as atelier-forest-light.

We are still not changing the version.sbt file to do it in a next PR to launch the next release, once done so, we must to change the plugins.sbt file so sbt-microsites will display the new light style.

Fixes #334

[calvellido]

We need to specify that Jekyll 4.0.0 at least is required to be installed on the system

[israelperezglez]

Good Job @AntonioMateoGomez , I add some comments:

• Links should have a :hover
• Could we pick another Theme for highlight.js?
• In the sidebar we could add a :hover in the li a
• #masthead .text-center doesn't work in responsive. width: 750px --> max-width: 750px

[calvellido]

Regarding the page layout:

https://47deg.github.io/sbt-microsites/docs/layouts.html#page-layout-and-menu-partial-layout

We need to:

• Either specify that the page layout is not supported for the new default light theme. Also, this will be important to remark in any version release announcement, as it could break sites like Scalaz, Cats, or fs2 ones.

• Either support this layout on the new default theme. Maybe we can even simply make a quick translation of the new home including the horizontal navbar.

Thoughts on this @juanpedromoreno, @AntonioMateoGomez, @israelperezglez? If we had to vote I'd try to go with the latter option 🙂

[calvellido]

This makes all light Home layouts include a menu when they shoudln't 🤷‍♂️

[calvellido]

Yes, it's present in the old pattern theme, and ideally we could be fixing it there too, but at least we shouldn't be dragging those bugs to the new themes.

This is what this generates in a usecase of a single Home layout, an unnecessary empty ul: