keys: headings are a key away, read or written

`]]` and `[[` jump to the next and previous heading of the open document,
in the reader and in the editor, where vim's section motions mean nothing
useful in markdown. Past the last heading they go on to the end, as vim's
do. A heading a jump has just parked at the top counts as the one being
read, so pressing again moves on.

`gO` (or `:outline`, `:toc`) lists the headings in an overlay that opens on
the section being read or the one the cursor is in: `j`/`k` walk it,
`Enter` jumps there. It is the margin's outline for when the pane is too
narrow to show one, and for hands that stay on the keys. In the editor the
list comes from the `#` lines of the text, code fences left out, and a pick
moves the cursor; in the reader it comes from the rendered headings,
markdown files included.

web/src/lib/Help.svelte

@@ -12,6 +12,8 @@
     ['j / k · ↓ ↑', 'move down / up the list — scroll the document when it has focus'],
     ['gg / G', 'first / last row, or top / bottom of the document'],
     ['Ctrl-D / Ctrl-U', 'half a page down / up the open document'],
+    [']] / [[', 'next / previous heading of the document — in the editor too'],
+    ['gO', 'the headings as a list: j/k, Enter jumps there'],
     ['Enter · l', 'open a document, or enter a folder — on an open keystore or sealed file, Enter asks for its password'],
     ['h', 'from the document to the list, from the list up a level'],
     ['o', 'new document here, straight into insert mode'],