Add duration of instances to the documentation

Problem / Motivation

It would be good if the instance duration would be shown on the home page.

Steps:
1. Go to simplytest.me

Desired result:
How to use it? contains:
1. Start typing a project's name or enter a shortname directly.
2. Choose your preferred version. Multiple versions of Drupal are supported.
3. Hit the button (wait until your sandbox is ready) and test it!
4. Your default username/password are admin/admin.
5. Your instance will be available for [duration].

Desired result:
How to use it? contains:
1. Start typing a project's name or enter a shortname directly.
2. Choose your preferred version. Multiple versions of Drupal are supported.
3. Hit the button (wait until your sandbox is ready) and test it!
4. Your default username/password are admin/admin.