doc: improve CSS for home page grid

Update the ACRN logo size to remove specifics in the CSS.  (Grid images
should be ~100px high for the grid.)

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/index.rst

@@ -17,7 +17,7 @@ through an open source platform.
    <ul class="grid">
        <li class="grid-item">
            <a href="learn.html">
-               <img alt="" src="_static/images/ACRNlogo.png"/>
+               <img alt="" src="_static/images/ACRNlogo80w.png"/>
                <h2>Learn about<br/>ACRN</h2>
            </a>
            <p>Overview, architecture,

doc/static/acrn-custom.css

@@ -232,8 +232,8 @@ kbd
 }
 
 .grid-item img {
-    max-width: 50%;
-    max-height: 50%;
+    /* max-width: 50%;
+    max-height: 50%; */
     margin-bottom: 0.7rem;
 }