Add notes on configuring GitHub repo after creation

There are broken URLs in the link check CI until these are done ([e.g.](https://github.com/DiamondLightSource/odin-fastcs/actions/runs/9774084697/job/26981591914)):
- Select the `gh-pages` branch in `Settings > Pages` to deploy the documentation
- Click get started in Discussions

Start by locating the repository documentation or post-creation setup instructions; the issue does not name a file. Document selecting the gh-pages branch under Settings > Pages and enabling Discussions, then run the link-check CI to confirm the broken URLs are resolved.

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