docs: add public collections gallery howto

[ikreymer]

Adding a 'howto' section on how to enable public collections gallery, extracted from blog post - to be linked from the blog post.

[Shrinks99]

From @Shrinks99 in Discord chat:

I think I'd rather surface this type of how to content in an admonition closer to the information in the docs about what the guide is covering. This isn't a particularly long how-to guide and we already have a page that is linked from the public collections docs that — in tandem with the in-app copy — should give a decent path for users to follow?
We also have a section on sharing collections already: https://docs.browsertrix.com/user-guide/presentation-sharing/#sharing-collections so that part of the guide is duplicate content