Docs: make Snippets TOC effective and split Contributors' Guide at sections

Docs: make Snippets TOC effective and split Contributors' Guide at sections

* make Snippets TOC anchors for individual snippets effective by creating
nodes for each snippet; this produces a lot of duplicate nodes but
it works in texi2html; support for makeinfo should be dropped
after this change though;

* add support for splitting HTML Texinfo output at section level;
thanks to Reinhold Kainhofer for the idea and a part of the patch.
(cherry picked from commit 2225db4c97bf1d842860658874d66b54ef3c8f5d)