Sectional table of contents in HTML output

[Gavin Smith]

I would like a way of having a menu in each HTML file that lists the 
section names of subordinate nodes.  This would replace the menu that 
already exists which is generated from the @menu block in the source.
I don't believe there is any way to do this currently with texi2any.

The reasons that this would be useful:

* The section names are better than the node names for saying what 
the node is about.
* The second column in the menu is unnecessary.  A whole row of the menu 
may not fit in the width of a small window leading to line breaking in 
the node name.
* Web browsers on mobile devices don't always cope well with the <table> 
element used for the menu and may render it with a tiny screen font.

It could be enabled with a customization variable like MINI_TOC or 
SECTION_TOC.  SHOW_MENU would be used to avoid outputting the menus.

I don't think that users should need to think about node names when 
browsing HTML files.

As far as I understand it, the double column menu format comes from the 
Info mode in Emacs.  At first, the standard way to select a menu entry 
was to type in the name of the menu entry using the "m" command.  Hence, 
short node names were preferred which differed near the start (for 
completion).  The extra description was needed because the entry names 
were short.  But people using web browsers are nearly always clicking on 
the link with a mouse, and not typing in a link name.  The double column 
format in HTML pages is a relic.