Infer the document title for `mld` files

[rizo]

Currently the mld filename is used as a title for the generated HTML document. This is not ideal because the name is an "artefacts of the underlying producing technology" and can be defined with the {0 ...} heading (or the first heading) in the content.

The implementation needs to take into account the fact that multiple mld files can be combined to produce a single document (equivalent of a LaTeX chapter, for example).

Several alternative approaches were discussed in https://github.com/ocaml/odoc/pull/217 for a potential solution.

[github-actions[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.