Add recipe for adding buffer diagnostics to the `tabline` with Heirline

### Is your feature related to a problem?

The `bufferline` provided ways to configure a UX wherein:
- The diagnostics of each open buffer would be indicated in that buffer's tabline entry
- The overall count of each category of diagnostics (errors, warnings and hints) in all open buffers in the workspace can be displayed in a custom area to the far top right corner of the tabline

### Describe the new feature

Provide the aforesaid configuration options in AstroNvim (if not in baseline, at least as a custom recipe in the documentation).