Find alternative to sendsafely

[TomOnTime]

documentation/advanced-features/byo-secrets.md line 172 still points contributors at https://transfer.secretoverflow.com/u/tlimoncelli. That's Stack Overflow internal infra and presumably won't be reachable for the independent project anymore.

[imlonghao]

How about the traditional GPG or age for encryption?

[TomOnTime]

I did some checking: SendSafely has a free tier.

Next step: verify it has the features we actually use.