docs: corrections for Nx page (#10019)
### Description
@jamesHenry pointed out some inaccuracies on the Nx page, so correcting
them here!
I apologize for these. I'm not an Nx expert and, as made more clear in
this PR, the sections in the "Why switch?" come from Nx users looking to
migrate. I tried my best to bring together their words, but missed the
mark. Namely, I learned that Nx's plugins are optional to use and I
mis-characterized how Nx's starter templates work.
