Write upgrade documentation for 9.3

Write the documentation to upgrade to 9.3. We need this as much complete as possible by the feature freeze (and release branch creation) because Mathieu needs it to upgrade intrapre. It can be updated after feature freeze with upgrade instructions from features complete at the last minute or issues we may discover when intrapre is upgraded.

Pages to be updated are listed below.
Upgrade from LTS 2016 to 9.2:

• Rename page into Upgrade from LTS 2016 to 9.3 and update file name accodingly (+ add redirect, see below)
• Add a section "From 9.2 to 9.3". Put content into excerpts to be reused in "Following Fast Tracks" page.

Upgrade from LTS 2016 following Fast Tracks:

• Add excerpts from page "Upgrade from LTS 2016 to 9.3" in relevant sections

Redirects page:

• Add redirect :

  ^/nxdoc/upgrade-from-lts-2016-to-92/?$: /nxdoc/upgrade-from-lts-2016-to-93/