Improve a11y

[qpavy]

Question	Answer
Branch?	develop
Bug fix?	yes
New feature?	no
Breaking change?	no
Tickets	Fix #PRB0041163
License	BSD 3-Clause

• [x] Try to keep pull requests small so they can be easily reviewed.
• [x] Commits are signed-off
• [x] Only FR translations have been updated
• [x] Branch is up-to-date with target branch
• [x] Lint has passed locally
• [x] Standalone app was ran and tested locally
• [x] Ticket reference is mentioned in linked commits (internal only)
• ~~[ ] Breaking change is mentioned in relevant commits~~

Description

• add aria-live region for assistive technology
• add Skip to main content link in container
• improve language switcher a11y
• improve legacy sidebar a11y
• improve navbar a11y
• improve notifications sidebar a11y
• improve Account Menu a11y

Related

• OLD PR: https://github.com/ovh/manager/pull/9716