[avr-chat] small bugs in the AVR libc online docs

[Britton Kerin]

The index (Alphabetical Index link).  It seems to be mostly empty.
Also I think the index should go under the Library Reference link
like how glibc/gtk and others do it, you just sort of expect to find
it at the bottom.  Or an extra link to it there.

Love to avrlibc as always,
Britton