Having old ADRs makes it harder for newcomers, that have to navigate
through many ADRs and spot the ones that were superseded by others. We
can save the mental burden by just moving the old ones to a deprecated
folder
This PR puts our contributing guidelines in the sidebar of the unleash
documentation. Currently there was no way of navigating to them easily,
which made our contribution guides and ADRs less useful. This PR adds
them to the sidebar as their own category, and adds an ADR for domain
centric language.
Co-authored-by: Thomas Heartman <thomas@getunleash.ai>
