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.

docs/wiki/Files-and-Search.md

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+# Files and search
+
+## Files
+
+A file is a node like a document, only with bytes instead of text. It sits in a
+folder beside the notes about it, shows its size in the list, and `Enter` opens
+it.
+
+Put files in:
+
+- drop them onto the list;
+- `:upload` picks files for the folder you are in;
+- `:import` uploads a whole folder, and `:import!` a zip
+  (see [Not losing things](Not-Losing-Things.md#import));
+- copy them in over the [SFTP mount](SFTP.md).
+
+Each upload is written to disk as it arrives, never held in memory, so the only
+limit is `WORKSPACE_MAX_UPLOAD_MB` (1024 by default) per request.
+
+### What opens in place
+
+- images, video and audio;
+- PDFs, in the browser's own viewer;
+- text and source code of any kind, with line numbers, up to the first
+  256 KiB;
+- markdown, rendered, with `:source` to see it as written;
+- CSV and TSV, as a table of the first 500 rows;
+- certificates, keys and keystores (`.pem`, `.crt`, `.cer`, `.der`, `.csr`,
+  `.key`, `.p8`, `.p12`, `.pfx`), read on the server.
+
+![A CSV file as a table](../images/file-table.png)
+
+A certificate shows its subject, issuer, validity, key, names, usage and
+fingerprints, and which key belongs to which certificate. A `.p12` or an
+encrypted key asks for its password, which opens it for that one look and is
+not stored. Nothing of a private key is sent back but its algorithm and size.
+
+![A certificate read on the server](../images/file-certificate.png)
+
+Anything else is a download: `:download` (`:dl`) fetches the file under the
+cursor, and `gx` opens it in a browser tab of its own. Files are renamed with
+`:name`, moved like documents, and deleted into the trash with `x`.
+
+Uploads are only ever served as types that cannot run in the browser. An HTML
+file or an SVG, for example, downloads instead of opening.
+
+## Search
+
+| key | searches |
+| --- | --- |
+| `/` | the space you are in |
+| `Ctrl-P` | every space |
+
+Results appear as you type, ranked by relevance, each with the space and
+folder it is in. `Tab` or `Ctrl-N` / `Ctrl-P` walk them without leaving the
+line, and `Enter` opens the one lit. Afterwards `n` and `N` open the next and
+previous result beside the list, and `:noh` clears the search.
+
+![A search that found a line in a log file](../images/search.png)
+
+Each word you type matches the start of a word, so `inge` finds "ingest". All
+the words must be there. Punctuation is ignored.
+
+### Inside files
+
+A search reads the text files in the tree as well as the documents. A file
+counts as text by its type, or when its first 8 KiB read as UTF-8 with no NUL
+byte, which covers `.log` files and Makefiles. The first 256 KiB of each is
+indexed, the same part the viewer shows. A file found by its text shows the
+words around the match under its name.
+
+The text is read when a file arrives, by upload, import or the SFTP mount,
+again when its bytes change, and at start for any file still missing it. PDFs,
+office documents and images are not read.
+
+A sealed file is never indexed; see [The vault](Vault.md). Sealing a file drops
+its text in the same step, and decrypting it reads it again.