Moved general FAQ to top-level index page.

Added entries to FAQ related to speeding up Theano function compilation and out-of-memory errors.
Can you add a paragraph to something like this:

GPU don't have virtual memory, that is why they request that all allocation be assigned to a continuous memory region. As CPU support virtual memory, they don't have this requirement. When changing the size of a shared variable, it can cause memory fragmentation on the GPU, which make it harder to find big continuous memory region for following memory allocation.