[dev doc] Update proof engine docs, fixes #6640
[dev doc] Update proof engine docs, fixes #6640

We update the docs for the removal of `Sigma` and the deprecation of
`enter_nf`.

JasonGross/coq,coq/coq,Zimmi48/coq,ejgallego/coq,gares/coq,SkySkimmer/coq,ppedrot/coq,ppedrot/coq,maximedenes/coq,SkySkimmer/coq,JasonGross/coq,ejgallego/coq,JasonGross/coq,silene/coq,Zimmi48/coq,ppedrot/coq,gares/coq,JasonGross/coq,maximedenes/coq,maximedenes/coq,coq/coq,silene/coq,Matafou/coq,maximedenes/coq,ppedrot/coq,SkySkimmer/coq,coq/coq,gares/coq,SkySkimmer/coq,gares/coq,Matafou/coq,silene/coq,Matafou/coq,ppedrot/coq,gares/coq,ejgallego/coq,ejgallego/coq,maximedenes/coq,Zimmi48/coq,Zimmi48/coq,coq/coq