Add new graphics to documentation (#762)

Updates README and docs homepage with new logo

Author: edwardchalstrey1

README.md

@@ -1,7 +1,4 @@
-<h1 style="text-align: center;">
-<img src="https://raw.githubusercontent.com/gambitproject/gambit/master/doc/_static/gambit.png"
-     width="300" alt="Gambit logo">
-</h1>
+![Logo](doc/_static/Gambit_logo_landscape_green_font_no_outline.png)
 <br/>
 
 ![Latest stable version](https://img.shields.io/github/v/release/gambitproject/gambit?filter=!*a*&label=Latest%20stable%20version)

doc/_static/Gambit_logo_landscape_green_font_no_outline.png

@@ -14,9 +14,9 @@
 
 <div style="background-color: white; text-align: center; padding: 10px 10px 15px 15px; align-items: center;">
     <a href="https://www.gambit-project.org/"><img src="{{
-    pathto("_static/gambit.png", 1) }}" border="0" alt="Chadwick" style="width: 12%;"/></a>
+    pathto("_static/Gambit_logo_landscape_green_font_no_outline.png", 1) }}" border="0" alt="Chadwick" style="width: 12%;"/></a>
 
-    <h4 style="color: #cc0000; margin: 0 auto; text-align: center; width: 100%;">Software tools for game theory.</h3>
+    <h4 style="color: #1a9e7e; margin: 0 auto; text-align: center; width: 100%;">Software tools for game theory.</h3>
 </div>
 {{ super() }}
 {% endblock %}