Add sustainability in the strategic goals and simplify structure

Author: dpiparo

_data/authors.yml

@@ -17,7 +17,7 @@ René Brun:
                  a dead-end line, he created the ROOT system while working for the NA49
                  heavy ion experiment at the SPS. Rene has led the ROOT project from
                  1995-2010."
-  tag         : "founder"
+  tag         : "previous"
   avatar      : "/assets/images/RB.jpg"
   affiliation : "CERN"
   email       : "Rene.Brun@cern.ch"
@@ -39,7 +39,7 @@ Fons Rademakers:
                 to high performance parallel computing using PROOF. Fons took over from
                 Rene Brun as ROOT project leader in 2011 and has led the project till
                 2014, culminating in the release of ROOT 6."
-  tag         : "founder"
+  tag         : "previous"
   avatar      : "/assets/images/FR.png"
   affiliation : "CERN"
   email       : "Fons.Rademakers@cern.ch"

_data/navigation.yml

@@ -206,8 +206,6 @@ about:
      url: about/license
    - title: Versioning
      url: about/versioning
-   - title: Project Founders
-     url: about/project_founders
    - title: Team
      url: about/team
    - title: Previous Developers

about/index.md

@@ -10,7 +10,7 @@ on high-energy physics. Every day, thousands of physicists use ROOT applications
 to analyze their data or to perform simulations. 
 
 Our **strategic goal** is to provide a unified software package for the storage, processing, 
-visualisation and analysis of scientific data that is reliable, performant and supported, 
+visualisation and analysis of scientific data that is reliable, performant, supported, and sustainable
 that is easy to use and obtain, and that minimises the computing resources needed to 
 achieve scientific results. The success of experiments and all ROOT users at large is 
 our priority.