Formatter: update installation guide

[Boshen]

Need to update formatter installation guide to be in the same style as the linter.

[Boshen]

@connorshea would you be keen to take over this one?

[connorshea]

Do you want to basically have it follow the structure of the linter installation guide? I can see about doing that sometime this week.

[Boshen]

Do you want to basically have it follow the structure of the linter installation guide? I can see about doing that sometime this week.

yes, the current installation guide is lacking too much useful information.