Merge scala 2 and scala 3 themes and guides

[bishabosha]

Reverse splitting the documentation website into two distinct themes.

• merge the two navigation bars
• use only a single colour for the theme
• remove the "Work in Progress" notice from scala 3 pages
• merge the scala 3 guides with the "guides and overviews"
• add notices to a limited number of pages that are "version specific" to either scala 2 or scala 3.
• merge the "Scala 2" and "Scala 3" sections of the index page.
• set up "new-version" metadata for every page in scala 2 book, add a new theme for outdated pages
• move scala 2 book to "guides and overviews"
• add notice to scala 3 and scala 2 book that we are in process of merging them.

[julienrf]

Thank you Jamie!

I think it would be good to always show the navbar, like it was the case before the Scala 2 / Scala 3 split (see here).

[julienrf]

Thank you for addressing all the comments! The result looks good to me, I only left a couple of minor comments.

[kazanzhy]

Just subscribing to receive the notification and prepare a Ukrainian translation of these changes