Update accessibility document

[SiniCode]

The current accessibility document was created in 2021. Time for update...

[SiniCode]

Detected issues (marked when fixed):

• User menu is not keyboard accessible
• Main navigation that becomes a hamburger with large zoom is not navigable with keyboard.
• Linked images do not tell where the links take
• External links do not convey the information about external redirect for screen readers
• Navigation bar is not identified as such for screen reader
• Language selection is not clear with screen reader
• Skip links are not available
• Focus indicator very bad on some pages/elements -> should be clear everywhere
• Missing labels
• Broken aria references
• Some components do not behave nicely with large zoom
• Contrast issues, especially in summary