Add manual build trigger to docs CI action

[melpadden]

The Portal CMS is used to create the nav header over top of the docs themselves, which are generated using a Docusaurus build. Ditto the footer. So, any change to the header or footer can't be reflected in the compiled/published site until someone makes a change to the docs themselves.

We can't trigger a CI build with any other mechanism outside of a spurious PR and build.

We need a simliar trigger to the one which exists in the Portal repo to allow us to trigger a build and update the header and footer accordingly