Fix table widths in lecture overview

Quarto derives the relative cell widths from the number of dashes in the line separating the table header from the table body [0]. As a consequence, the date column was very thin causing line breaks.

This commit completely removes all efforts to make the table look nice in ASCII in favor of a proper rendering as well as easier editing.

[0] https://quarto.org/docs/authoring/tables.html#column-widths