Help: Simplify top-level index organization

Revise the organization introduced in commit 2c7cd95c (Help: Organize
top-level index, 2013-10-28) to drop "Other Manuals" and put them in
"Reference Manuals" because the distinction between them has blurred.
Perhaps a better breakdown of the reference manuals will emerge in
the future.

While at it, sort the reference manual toctree by name.
Merge topic 'doc-organize-index' into next

12862e9 Help: Simplify top-level index organization