doc: generate NIC overview table from ini files

doc: generate NIC overview table from ini files

Convert the NIC feature table in the overview doc into a set of ini
files and add functions into the Sphinx conf.py file to auto-generate
them back into an RST table.

The reason for doing this is to make it easier for PMD maintainers to
update the feature matrix that makes up the table and to avoid
frequent and hard to resolve conflicts in doc/guides/nics/overview.rst.

A NIC/PMD feature matrix is now an ini file like the following:

    $ head doc/guides/nics/nic_features/i40e.ini
    ;
    ; Features of the i40e network driver.
    ;
    [Features]
    Link status          = Y
    Link status event    = Y
    Rx interrupt         = Y
    Queue start/stop     = Y
    ...

The output RST table matches the existing table with the column
headers sorted.

Signed-off-by: John McNamara <john.mcnamara@intel.com>
Tested-by: Ferruh Yigit <ferruh.yigit@intel.com>