Allow for cross-page bibliographies

[choldgraf]

Currently, a bibliography with references shows up automatically for pages that have DOI markdown links or {cite} roles on their page. However, these are page-specific. Many multi-page documents often include a single document-wide bibliography with all of their cited works.

Proposal

Make it possible to include a bibliography / references section that includes every citation used across any page in a MyST multi-page document. Ideally this would let you select subsets of pages arbitrarily, but a single "show every reference used" list would be fine.

I believe that sphinxcontrib-bibtex uses a directive like:

```{bibliography}
```

For this, and also uses this to expose different citation styles.