Add mention to OE FPGA Aquisition Board in docs for Rhythm plugins

[ChucklesOnGitHub]

• added mention to the OE FPGA Acquisition board plugin, until we unify them (it doesn't warrant a separate plugin page sicne they work in the same way)
• removed mention about upcoming support for other Intan hardware - I actually don't know the status of this but it took up space
• added brief comments that sampling rates may differ across devices since the new FPGA has a couple less option in the drop-down menu.