arc42 template (and some websites) are not accessible for visually impaired people

[gernotstarke]

a user from Australia sent the following remarks (and I'm deeply sorry we didn't care earlier!):

There are a number of cases where information seems to be contained in images throughout the document, but is not adequately described in alt text, for example:
https://docs.arc42.org/section-1/
Under heading 1.2, in Content:
The ISO 25010 standard provides a nice overview of potential topics of interest:
ISO 25010 categories of quality requirements

The alt text “ISO 25010 categories of quality requirements” does not tell me what those requirements are, and I have no other >source of this information on the page.

In addition, this page contains almost all headings at level 3. I would expect something more like
Heading Level 1: ARC42 Documentation
Heading Level 2: 1. Introduction and Goals
Heading Level 3: 1.1 Requirements Overview
Heading Level 4: Contents
Heading Level 4: Motivation
Etc

In this way the headings promote the structure of the document, and are browsable, in hierarchy, by screen reader users, allowing quicker access to the content, and supplying that structure information.