Support reading of title from H1 element

[mcepl]

Describe the feature you'd like to request

(originally filed as ashwinvis/myst-reader#48, but it probably belongs more here)

The Markdown module can substitute title value by the value of the first H1 element present in the body of text. That makes the Markdown document more generic and less bound to the particular reader. Would it be possible to extend MyST-Parser to be able to do it as well, please?

Describe the solution you'd like

The page would be valid even without title: express metadata, and its value could be (optionally?) sourced from the first H1 element.

Describe alternatives you've considered

Not applicable

[ashwinvis]

I have to look more into it, but I suspect it is already supported. Pelican has a way to handle the title: metadata and I pass it using this _process_metadata method.

https://github.com/ashwinvis/myst-reader/blob/46b191630097e05d13bd3acf61048cc8c2514276/pelican/plugins/myst_reader/myst_reader.py#L274-L293