doc: add hugepage mapping details

doc: add hugepage mapping details

Hugepage mapping is a layer of EAL malloc builds upon.
There were implicit references to its details,
like mentions of segment file descriptors,
but no explicit description of its modes and operation.
Add an overview of mechanics used on ech supported OS.
Convert memory management subsections from list items
to level 4 headers: they are big and important enough.

Signed-off-by: Dmitry Kozlyuk <dkozlyuk@nvidia.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>