Condense and clarify v3 onion services migration docs

[rocodes]

Status

Ready for review

Description of Changes

• Description:

• Remove initial v3 preamble and reference glossary for further details about onion services

• Clarify onion services migration requirements/consequences for failing to migrate

• Link to glossary for clarification of terms

• Tweak 'onion service' subheading in glossary, for clarity

• Fixes Update v2/v3 Onion Page Text #122

Testing

• manual review + linting

Release

• n/a / with 1.7.0

Checklist (Optional)

• Doc linting (make docs-lint) passed locally
• Doc link linting (make docs-linkcheck) passed
• You have previewed (make docs) docs at http://localhost:8000

[rocodes]

cc @ninavizz : Thank you for your feedback, welcome your review!

[rocodes]

I changed this because previously, it said "Onion Service v2" and then the first sentence started "The third generation of onion services (v3) provides stronger... " etc; I found that slightly confusing.

[rocodes]

This action will not affect sources or journalists, since the v2 onion
URLs will continue to work

I feel like we need to say something like this so that administrators are aware they can take the first migration step without having to coordinate with journalists.

[eloquence]

Makes sense, I just tweaked this slightly to avoid misinterpretation of "will continue to work".

[rocodes]

I've left this in about as decent shape as I can :) I'm off for the rest of the day but @eloquence has said he'll push any changes he sees fit due to time-sensitive nature.

[eloquence]

Very sensible changes, thanks @rocodes (and @ninavizz for suggestions incorporated here). I only pushed a tiny commit, so will go ahead and merge.