Add recipe for adding buffer diagnostics to the `tabline` with Heirline about docs HOT 1 OPEN

This is a relatively similar response as I gave to AstroNvim/AstroNvim#2112 so I'll keep it small. This is not in the scope of what we want to provide out of the box and is implementable through our Status API. There are many users on the Discord that actually do this successfully so that's a good resource for asking questions and seeing what others have done.

I do like the idea of this being a recipe in the documentation and could even be added as a feature in AstroCommunity to be easily added (similar to other recipes such as the VS Code winbar and the mode text in the statusline). So I will just migrate this issue over to the documentation and add a "help wanted" as I don't when I will have the time I will have to implement this for you and write documentation for it. Thanks for the suggestion!

from docs.