Add onion services apps to the glossary

Description

There are some terms in https://onionservices.torproject.org/apps/web/ that we should add to the glossary.

I have seen: onionspray, oniongroove, onionprobe, onionmine, onionbalance

But maybe there are more.

Guidelines

• Present an official definition.
• Simple, canonical definition (like a phrase).
• Also spelling.
• With links for more info (like official docs).

Tasks

Add glossary entries for the following terms, by opening merge requests (content to be added in this folder, in this branch):

• Onionspray.
• Oniongroove.
• Onionprobe.
• Onionbalance.
• Onionmine.

Time estimation

• Complexity: very small (0.5 day)
• Uncertainty: low (x1.1)
• Reference (adapted)