Migrated commands individually to work as GitHub actions with a
[`workflow_dispatch`](https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch)
event.

This will not disable the command-bot yet, but it's the first step
before disabling it.

### Commands migrated
- [x] bench-all
- [x] bench-overhead
- [x] bench
- [x] fmt
- [x] update-ui

Also created an action that will inform users about the new
documentation when they comment `bot`.

### Created documentation 
Created a detailed documentation on how to use this action. Found the
documentation
[here](https://github.com/paritytech/polkadot-sdk/blob/bullrich/cmd-action/.github/commands-readme.md).

---------

Co-authored-by: Alexander Samusev <41779041+alvicsam@users.noreply.github.com>
Co-authored-by: Przemek Rzad <przemek@parity.io>