Add doc about SVIF (system verilog interface)

[yportne13]

for this pr: https://github.com/SpinalHDL/SpinalHDL/pull/1364

[andreasWallner]

FYI: while WIP please open as a draft - then it doesn't show up in the inboxes of the maintainers to review.

[Dolu1990]

Hi,

One thing which is important i think, is to specify in the doc that this feature may be subject to changes (experimental) That will let us more wiggle room for future updates :)