[accessibility] automated checker result

[brichet]

@balajialg highlighted the fact that there have been some improvements about accessibility with Notebook 7: berkeley-dsep-infra/datahub#3885.

He provided a list of problems found by the Axe Decque test tool that should be fixed.

These remaining errors have several levels of severity and come from different projects (Codemirror, JupyterLab, Lumino and Notebook).

Critical/serious

JupyterLab:

• the responsive button in toolbar that have no aria-label
• several insufficient color contrast
• aria-label on div without role

Codemirror:

• missing aria-label to div with role
• several insufficient color contrast

Moderate/minor

Notebook

• Missing landmarks in document: https://dequeuniversity.com/rules/axe/4.4/landmark-one-main?application=AxeChrome

Lumino:

• wrong role (button) to title of accordion panel
• wrong role (checkbox) to toggleable items of the command palette