doc: ariane-fpga: Fix doc styles

Various styles fixes including:

- satisfy the 80 character per line rule as much as possible
- make title underline the same length as the title itself
- remove the redundant FPGA (was FPGA FPGA SoC)

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>