add initial usecases to homepage

[RossKen]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

Is your Pull Request linked to an existing Issue or Pull Request?

NA

Give a brief description for the solution you have provided

First draft of adding use cases to the docs. I think this will be helpful to keep a centralised list of use cases (or at least the ones we can provide a public link to). A couple of point I would appreciate some thoughts on:

• Currently just on the docs homepage, not the gitbhub readme. Is this an issue? Should we have it in both places? The structure would have to change if we wanted consistency
• Where should it live on the homepage? Is it visible enough where it is at the moment?
• Are there any examples we are missing?

PR Checklist

• Added documentation for changes
• Added feature to example notebooks or tutorial (if appropriate)
• Added tests (if appropriate)
• Updated CHANGELOG.md (if appropriate)
• Made changes based off the latest version of Splink
• Run the linter