Allow to set default title in metadata

[ahanak]

Allow to set the default title for each metadata block.
This allows to omit the title when creating divs.

My use case:

---
pandoc-beamer-block:
  - classes: [good]
    type: example
    title: "Good Code"
  - classes: [bad]
    type: alert
    title: "Bad Code"
---

::: bad :::
Some Negative Example
:::

::: good :::
Some Positive Example
:::

Please note that I disabled pre-commit hooks as I wanted to keep that PR simple. If you want, I can include the modified uv.lock file and fix the import order in docs/conf.py.

[chdemko]

Thanks for this PR. It's very usefull I think