docs: a guide, a wiki and screenshots

The README grew a section per feature and is a good reference, but nothing
said where to begin. docs/wiki now has a getting-started page, a guide that
walks through one day with the workspace and explains each part where it comes
up, and a page per part of the app, each with screenshots of what it
describes.

Three pages are read out of the code rather than written, so they cannot drift
from it: the keys and commands from Help.svelte and commands.ts, the same lists
`:help` draws; the settings from .env.example, which gains a heading for the
vault, trash and history settings so they no longer sit under mail; and the
API from the README's table. `make wiki` writes them, and `--check` fails when
one is stale or a link between pages leads nowhere. `--publish` lays the pages
out for a GitHub wiki: links without `.md`, images beside them, `[[` escaped so
the wiki does not take a command's help text for a link to a page.

The screenshots are taken by `make screenshots`: the debug binary on a
temporary data directory, with a password made up for the run, filled with an
example workspace over the API, and a headless Chromium driven over the
DevTools protocol through the views the docs show. Nothing outside the
temporary directory is touched but docs/images, and the pictures are cut to
256 colours, a third of their size, which a dark UI does not show.

The README opens with the banner, a CI badge, the guide, the features grouped
by what they are for, and six of the screenshots.

.env.example

@@ -101,6 +101,8 @@ WORKSPACE_DIGEST_AT=sun 18:00
 # Which of the four kinds may be sent. Drop one from the list to switch it off.
 WORKSPACE_MAIL_KINDS=reminder,card_due,digest,login
 
+# --- Vault, trash and history ------------------------------------------------
+
 # Minutes the vault stays open with nothing using it, before it locks itself.
 # It also locks on logout and whenever the server restarts.
 WORKSPACE_VAULT_IDLE_MIN=15