Revive `source:3.1-migration` documentation

[adpi2]

Add a "Dotty 3.1 Migration Mode" page to document the -source:3.1-migration incompatibilities.
Move each description from this page to the corresponding readme in incompat-3.1 then include it in the "Dotty 3.1 Migration Mode" page using the mdoc modifier (see example).

Add this page in a "Going further" section.