Minor correction to Applications.rst
(also testing the github rst editor)

This ports this commit from otb-documents which didn't make it in time
for the merge:
https://git.orfeo-toolbox.org/otb-documents.git/commit/aa31a5cf9a514f1f03cfb9b9102aa6afaeb4f78e

This proof reads the installation chapter and
improves cookbook introdutions to OTB applications and Monteverdi by not
talking about OTB history, older versions of otb applications or
Monteverdi but directly introducing them in their current state,
focusing on a user completely unfamiliar with OTB or any of its previous
versions.

Don't overwrite math role:
https://github.com/sphinx-doc/sphinx/issues/2927#event-773707455