Write the operator guide

[iderex]

The release only means something if somebody who has never seen this board can use
it in a few minutes. Write the page that makes that true.

The shape it has to have. Download one file. Verify its checksum, with the command
written out for each platform, because a checksum nobody knows how to check is
decoration. Run one verb. Read what it printed.

What the operator gets, said in one paragraph near the top so they can decide
whether to continue. The runner reads a checkout of this board and tells them
whether its records are in order and what it examined. It changes nothing, it sends
nothing anywhere, and it needs no privileges. Somebody deciding in thirty seconds
whether to run an unfamiliar binary is asking exactly those three questions.

Include the failure paths, because they are what people actually hit. What a
refusal looks like and how to read it. What the exit codes mean. What a run over a
tree with no experiments prints, since that is what the first run produces and a
reader needs to know it is a real result rather than a broken one.

The page names the notice, the licence and the privacy document, and says the same
thing about personal data staying on the host that the privacy document says. Saying
it in both places is deliberate here, because the operator guide is the only one of
the two that a downloader is certain to open.

Done when docs/operator-guide.md exists on the default branch, walks download,
checksum and first run, quotes the exact output of a first run against a fresh
clone, and explains every exit code the runner can return.

[iderex]

The page is on the default branch as of e5067b1, and most of the done-condition
is met by it:

git ls-tree --name-only origin/main docs/operator-guide.md
docs/operator-guide.md

It quotes the exact output of a first run against a fresh clone, naming the
commit that run was made at, and it quotes the run over a tree with no
experiments next to it, which is the one this issue asks for so that a reader
knows an empty result is a real one. It carries the listing, what a refusal
looks like with the rule name in it, and every exit code, explained by pointing
at docs/decisions/0011-the-exit-codes.md and by saying which invocation
produces each one. The three questions somebody decides on in half a minute are
in one paragraph near the top, each written as a limit with the test or the
record behind it rather than as an assurance.

What is left is the first clause. The page does not walk a download or a
checksum, because there is nothing published to download and nothing to
checksum, and the section where a reader looks for it says that rather than
leaving a gap. Whether this board publishes a downloadable artefact at all is
entry four of #46 and is open, so the two paragraphs that clause asks for cannot
be written without inventing the thing they describe.

One other thing the page states as an absence. It names the notice and the
privacy document, and it names the licence as missing, because there is no
licence file to point at.