docs/networking: fix formatting of Intel drivers documentation

The documentation of Intel drivers is missing the heading adornment for
document titles.

This causes the generated html to have TOC entries from these documents to
appear as top level TOC entries:

* Linux* Base Driver for Intel(R) Ethernet Network Connection
* Contents
* Identifying Your Adapter
* Command Line Parameters
  * AutoNeg
  * Duplex
  ...

Add overline heading adornment to document titles.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>