5917: Add a command to open docs for the symbol under the cursor r=matklad a=zacps

#### Todo

- [ ] Decide if there should be a default keybind or context menu entry
- [x] Figure out how to get the documentation path for methods and other non-top-level defs
- [x] Design the protocol extension. In future we'll probably want parameters for local/remote documentation URLs, so that should maybe be done in this PR?
- [x] Code organisation
- [x] Tests


Co-authored-by: Zac Pullar-Strecker <zacmps@gmail.com>