Improve, expand, and document keyboard shortcuts

Currently, users can annotate selected text by typing `a` to create an annotation or `h` to create a highlight. This meets WCAG guidelines but could use some improvement and new additions. For example:

- Enabling/disabling the browser extension
- Opening and closing the sidebar
- Moving from document to sidebar & vice versa
- Interacting with an annotation: Editing, Replying, Flagging, and Deleting
- Editing an annotation: toggle between preview and edit mode

Notes
- All of these commands should have some kind of modifier (CTRL or CMD). We should also ensure that these do not interfere with commonly used keyboard shortcuts in the browser or other assistive technology.
- Keyboard shortcuts should be documented in the KB

To Do:

- [ ] Make a list of "actions" someone would need to do within the app
- [ ] Propose a set of keyboard shortcuts for each of these options (might be helpful to research commonly used shortcuts to avoid)
- [ ] Meet with our contacts at Benetech to validate those suggestions
- [ ] Get this issue on our product backlog to be prioritized among our other work

Start by turning the listed annotation, sidebar, extension, and navigation actions into a complete action list, then research browser and assistive-technology shortcut conventions. Validate proposed shortcuts with Benetech and record the resulting proposal in the product backlog; completion requires agreement on the shortcut set and documentation needs.

Written by the indexing model from the issue text.