Rewrite the docs

Currently https://aiomonitor.aio-libs.org is from the old version.
Let's update to reflect the additions and changes from the aiomonitor-ng, such as:

- Update screencasts and screenshots
- Introduce the new commands: `where-terminated`, `ps-terminated`
- Introduce the changed method for adding custom commands to the monitor
- Describe the difference between `monitor.console_locals` and `locals` kwarg in the monitor constructor (`console_locals` is to inject the local variables after instantiation)