Add link on how to setup Github PAT in README.

> Thanks for suggesting that I try to install shiny first and I'll give it a go... but I was hoping that following the installation instructions I would have everything that I need (I used instructions at https://insightsengineering.github.io/teal.gallery/).
> Although you mention that PAT is beyond the scope of Teal, since it's mentioned in the documentation I think it may help novice users like me if there was some kind of link to where I could read more.
> Just so I'm clear, it's fantastic that the Teal community can make tools like this available for free and my hope is that tweaking the documentation can help (even) wider adoption

_Originally posted by @kinnersleyn in [#89](https://github.com/insightsengineering/teal.gallery/issues/89#issuecomment-2891981539)_

Start by opening the README and locate the existing installation or GitHub PAT guidance. Add a link explaining how to set up a GitHub PAT, then verify that the README link is clear and works.

Written by the indexing model from the issue text.