Add doc on how to change an machine's series

[hmlanigan]

to Getting things done.

[pmatulis]

Thanks for this. 🍰

Review:

• Some general guidelines are included that need to be applied.
• It's not obvious what is meant by "updating a machine series". At least one sentence should "set the stage".
• I'm left wondering how to update a machine series from 2.3 onward.

[pmatulis]

Notes should be in the format below. I've also reworded. Does it still make sense (I'm not sure)?

!!! Note:
    Charms may have to be updated in order to take advantage of a series change.

[hmlanigan]

change made. 'upgraded' used instead as that is closer to the related juju command.

[pmatulis]

We use syntax highlighting using a "type" with 'bash', 'yaml', 'json', and 'no-highlight' being the most common. We also do not include the prompt.

```bash
juju switch localhost-test:admin/default
```

We also separate input from output. Typically the output (which uses the 'no-highlight' type) is prepended by something like:

This will give output closely resembling:

Try being creative with the above. It gets tiresome really quickly by using the same everywhere. An option is to sometimes include the command (if it's simple like juju status for example) within the preceding text itself (e.g. "Output to the juju status command will look like:").

[hmlanigan]

Added the highlighting types. Tried to make the output more interesting?

[hmlanigan]

moved to 2.2 and updated title to match pr 2019.