Support overriding the file title in the Table of content

[nthiery]

trafficstars

As of Myst 1.2.7 a toc entry in myst.yml such as

   - file: bla.md
     title: The Bla

produces an HTML table of content with the title for this entry taken from bla.md. Instead, one would expect (as in JupyterBook) to have "The Bla" as title for this entry.

Coud be related to #1222 .

/cc @choldgraf