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.

README.md

@@ -77,6 +77,8 @@ it; in the document pane it is that document. In the browse pane:
 | `j` / `k` | move down / up, or scroll the document when it has focus |
 | `gg` / `G` | first / last entry, or top / bottom of the document |
 | `Ctrl-D` / `Ctrl-U` | half a page down / up the open document |
+| `]]` / `[[` | the next / previous heading of the open document, in the editor too |
+| `gO` | the document's headings as a list — `j`/`k`, `Enter` jumps there (`:outline`) |
 | `Enter`, `l` | open the highlighted entry; on an open keystore or sealed file, `Enter` asks for its password |
 | `o` | a new document here, straight into insert mode |
 | `O` | a new folder here |
@@ -103,7 +105,8 @@ mode, macros, `:w` to write. `:help` lists every ex command; the useful ones are
 `:w`, `:wq`, `:q!`, `:new [yyyy-mm-dd]`, `:date`, `:name`, `:share`, `:link`,
 `:upload`, `:media`, `:search`, `:backup` and
 `:set novim`. `:name` on its own moves up into the title; `Enter`, `Esc` or `↓`
-there goes back down into the text. The rest: `:today` opens today's entry, `:edit [id]` (or `:12`)
+there goes back down into the text. In the text, `]]`, `[[` and `gO` walk the
+headings as they do in the reader. The rest: `:today` opens today's entry, `:edit [id]` (or `:12`)
 opens a node by id, `:next`, `:prev`, `:first`, `:last` and `:random` walk the
 list, `:delete [id]` deletes, `:copy` puts the whole document on the clipboard,
 `:clear` drops the search, `:reload` re-reads the folder, `:stats` counts words