ADR: why we only use the disabled attribute in very specific cases (as little as possible)

[florian-sanders-cc]

Context

The disabled attribute has cause many issues related to focus management.

They are fairly complex and they led us to take very opinionated decisions + conventions:

• use readonly instead of disabled during API calls for form controls (except buttons),
• for cc-button, use waiting instead of disabled (and forbid using both at the same time) during API calls.

Reasons for these changes are tracked within issues which contain fairly detailed explanations but it's not very convenient for new contributors or even for us.
We should write an ADR to sum up our decisions + why we made these choice.
We should also probably update the contributing guidelines to add a section about what to do with buttons & form controls during API calls (+ link to the ADR to explain why).