self-host: document how to start a trial deployment over from scratch

[L4XB]

Goal

make down stops the stack and keeps the volumes — correct for a real
deployment, wrong for someone evaluating who wants a clean slate. Neither
Makefile nor deploy/community/README.md says how to actually discard the
data and start again, so the first thing an evaluator wants to do after a failed
experiment is undocumented.

Where

• deploy/community/README.md — the Quick start and Updates and scaling
  sections are the natural neighbours
• Makefile — optional; if you add a target, see the warning below

What to do

Document the reset: how to remove the volumes, what is destroyed with them
(database, uploads, figures, the erasure journal), and that this is not an
upgrade path.

If you add a make target for it, it must be hard to run by accident — a bare
make reset that silently drops a production database is a worse outcome than
no target at all. Require an explicit confirmation argument, and say in the
## description that it destroys data. If you would rather leave it as
documented commands only, that is a legitimate answer; say so in the pull
request.

How to verify

Run the commands you wrote, from a stack you started yourself, and confirm the
next make up really does start from an empty database. Say in the pull request
that you did.

Acceptance criteria

• The reset path is documented with the commands, in order
• What is destroyed is named explicitly
• The distinction from make down and from make update is clear
• bash deploy/community/test.sh passes if you touched any script