Move General Features to Getting Started

[codePau]

I think this should go at the beginning of the documentation, after a little introduction to the project. Features can be renamed as Architecture.
https://docs.decidim.org/en/features/general-features/

[andreslucena]

This needs to be reviewed for sure first, as it was written in 2018 and some "Feature expected" comments that I'm seeing aren't real for sure in 2022 😄.

cc/ @carolromero