Update the warning messages on older versions of the docs

[kathryn-ods]

Update the warnings on the versions of the docs <0.4

Currently only 0.1 has a warning about being an older version. We need to add this onto the 0.2 and 0.3 pages.

Note
You are not reading the most recent version of this documentation. 0.3.0 is the latest version available.

[kathryn-ods]

The version warning on 0.1 docs has been automated openownership/data-standard-sphinx-theme#67

So we need to make the same thing apply to 0.2 and eventually 0.3

[kathryn-ods]

See whether a message can be added within the theme

[kathryn-ods]

Possibly move this ticket to theme repo

[kd-ods]

I've just realised this issue is already in the theme repo: openownership/data-standard-sphinx-theme#81

Closing this, in favour of that one.