[Feature] Override doc title with the first markdown header

[joshknowles]

Problem description

Currently Fern Docs auto-generates the <h1> using the page name. This can be overridden in the front-matter, but it's not possible to use a markdown # without it rendering two <h1>s

Why would it be useful?

It is nice to use native markdown headers (i.e. # Header so that the files render nicely when viewed directly in GitHub and other markdown viewers. Currently when porting content over from another markdown tool, I had to go and remove all of the #s

Describe the solution (optional)

You could either disable the auto-generation of H1s or allow the first # to override the page name.