Improve technical documentation

[swissspidy]

Need to properly document architecture and design decisions, as well as blockers.

Including:

• SharedArrayBuffer / cross-origin isolation / iframe credentialless requirements and limitations
• Upload store is global -> implications for when there are multiple editors on a page
• Public-facing API vs. what's intended to be private

[swissspidy]

Overall upload queue logic:

flowchart TD
    A[Add new item to the upload queue] --> B[Analyze file and determine list of operations]
    B --> C{Has next operation?}
    C-- Yes --> D[Perform next operation]
    D --> C
    C -- No --> E[Remove from queue to finish upload]

Example operations for an image:

flowchart TD
    A[Add image to the upload queue] --> B[Analyze file and determine list of operations]
    B --> C{Is HEIF image?}
    C-- Yes --> D[Decode HEIF image]
    C -- No --> E{Is there a big image size threshold?}
    D --> E
    E -- Yes --> F[Scale down image]
    E -- No --> G{Compress image before upload?}
    F --> G
    G -- Yes --> H[Compress image]
    G -- No --> J[Upload image to server]
    H --> J
    J --> K[Generate thumbnails]
    K --> L{Is there a big image size threshold?}
    L  -- Yes --> M[Upload original to server, if the image was scaled down]
    L -- No --> N[Remove item from queue]
    M --> N