Missing instruction in manual for installation of why3 ide

In section "5.1.1 Installation via Opam" of the manual, it would be nice to include the installation instructions to enable the ide command, e.g.:

opam install why3-ide

added component: documentation label and removed 1 deleted label

mentioned in commit 2022cdee

closed with merge request !507 (merged)

mentioned in commit b3800652