Clarify usage of layouts

[mlennert]

Currently, there are different files in the layout directory. Some seem to be used, others not and when one looks at the files in the content directory one can find a very diverse usage of these layouts, probably stemming from massive copy-pasting for creating new pages (at least that's the case for me):

`grass-website/content$ grep --exclude-dir=news/ -R "layout:"

download/mac.en.md:layout: "os" download/docker.en.md:layout: "os" download/linux.en.md:layout: "os" download/data.en.md:layout: "data" download/addons.en.md:layout: "os" download/windows.en.md:layout: "os" about/history.md:layout: "general" about/elements.md:layout: elements about/brand.md:layout: "brand" about/license.md:layout: "overview" about/community.md:layout: "community" about/history/van-warren-notes.md:layout: "general" about/history/releases.md:layout: "general" about/history/web-evolution.md:layout: "general" about/history/gnu-release.md:layout: "general" about/credits.md:layout: "credits" contribute/development.md:layout: "devel" contribute/donations.md:layout: "general" contribute/community.md:layout: "contribute" contribute/sponsors.md:layout: "contribute" learn/books.md:layout: "overview" learn/overview.md:layout: "overview" learn/manuals.md:layout: "manuals" learn/tutorials.md:layout: "overview"`

I would need some guidance on this:

1. When do we need a specific layout file ?
2. If no specific layout is needed, which would be the main layout template ("general" ?)