Add migration instructions for v3 instances and clarify migration paths

[rocodes]

Status

Ready for review
Replaces #133

Description of Changes

• Description: Add backup and restore information for v3 instances. Add detailed hardware migration information. Include information on migration with preserve-tor-config flag.

Testing

• visual review

Release

• n/a

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]

@eloquence: I moved the instructions in v3_update_common out of the includes and back into the migration docs. They're now only used once (as opposed to being duplicated for v2+v3, and v3), and I think our future selves will thank us. I know it creates a big diff, but the changes in this PR are pretty much just the ones you requested.

[eloquence]

In case folks type those commands by hand (and to make them easier to read), I would suggest omitting the destination filename in all cp operations where it is not modified. So:

cp $SD_OLD/group_vars/all/site-specific $SD_NEW/group_vars/all/
cp $SD_OLD/tor_v3_keys.json $SD_NEW/
cp $SD_OLD/sd-backup-old.tar.gz $SD_NEW/
cp $SD_OLD/SecureDrop.asc $SD_NEW/
cp $SD_OLD/ossec.asc $SD_NEW/
cp $SD_OLD/journalist.asc $SD_NEW/

This is done further below for some of the commands.

[rocodes]

gotcha, fixed in 7f2b421

[eloquence]

Thanks @rocodes, this looks great to me. The comments I left are minor, and I'd be fine to merge as-is if push comes to shove, but see what you think. (We can tweak further during final QA as needed.)

[eloquence]

Looks great. With the revised release schedule, I'm hoping we'll have time tomorrow to step through a full migration following these docs, once #169 also lands.