on api page for experiments, crosslink DTN page for experiments

[opto]

this: https://developer.thunderbird.net/add-ons/mailextensions/experiments

could be referenced here:

https://webextension-api.thunderbird.net/en/91/how-to/experiments.html

and vice versa

[jobisoft]

Done: https://webextension-api.thunderbird.net/en/stable/experiments/introduction.html