Improve key rendering in documentation

[neiljp]

There are a few improvements we can make in the script which generates docs/hotkeys.md; some of the same changes could be added to the rendering of keys in the help menu too.

• [x] space-separated keys (page up/down) are rendered as separate keys in hotkeys.md - they should be one key
• [x] we may wish to capitalize or otherwise rename some keys, from those understood by urwid to how they should be rendered
• [ ] in particularly we should consider adding extra symbol text to some keys, eg. ←, ↑, →, ↓ (left ←?)
• [ ] we might discuss this, but it can be useful to show P as shift+p (or even shift+P?) [noted in #922]

Other ideas are welcome :)

update First two points fixed by d65cbee7406a3413946e78fb7fcff8ddc8875251 from #1479,