User Manual Abbreviations - `CSR` and `RVFI` in Section Names

Created by: silabs-robin

This PR adds (CSR) and (RVFI) to the corresponding section headers in the user manual.

The reason is that I several times have wanted to look up those things, hit ctrl+f and found nothing, having to stop for a moment and try to remember what we titled our section headers or scan through the list of sections until I find it. This change will alleviate some discontentment.

I have not tested this in any way. For those familiar with the formatting, you might visually recognize if it is correct or not.