Major Rework of plusstorage Documentation Section

[furtni]

Description

This PR introduces a major rework to plusstorage's section of the documentation.

I tried to group the changes into sensible commits to keep things comprehensible.
Changes are limited to /content/{de,en}/storage-backup.

• Set up directory structure to accommodate sub-folder for images
• Converted embedded base64 data to image files
• Formatted markdown files with Prettier and markdownlint
• Added two new pages that were created during this rework
• Fixed all regressions introduced by reformatting

Type of change

• Documentation update
• Bug fix (non-breaking change which fixes an issue)

Checklist:

Documentation

• My documentation follows the style, formatting and structure guidelines of this project ( Styling-Guideline )
• I have performed a self-review of my own documentation ( General-Guide )

Technical

• I have performed a self-review of my changes