Section headers

[jeanas]

reST requires section headers to be at least as long as the title. This is invalid:

Section title
============

Also, overline and underline must also be the same length. This is equally invalid:

##############
Title
#############

Sphinx is able to warn about the first, but not about the second. In the latter case, the title just gets dropped completely (which surprises me somewhat). At any rate, it'd be nice if we could check for it as it's a somewhat easy mistake to make.

[jeanas]

@JulienPalard Would you be OK with adding Docutils as a dependency? It's needed for its helper docutils.utils.column_width in order to take special Unicode widths like with combining characters and other weird Unicode stuff (if we want to care about that). It's going to be pulled in by Sphinx if we start using it anyway.

[JulienPalard]

In one hand it feels overkill to depend on docutils for ~3 lines of code, on the other hand it's going to be pulled anyway by Sphinx so OK.