docs folders cleanup

[alex-cit]

We should clean the docs structure a little bit to make the file structure easier to navigate (and make changes such as redesigns more effective):

• move content to folders + index.md files (so that there is less files in the section folder)
• if needed, cleanup the names, e.g. rename "install-input" to "install-mobile-app"

Be extra careful, as this leads to redirects.