Use better icons on home page and fix Slack icon (#654)

## Description of the changes
- update some material-ui icons to better match the features being
called out
- fix the size of Slack icon in the footer

Signed-off-by: Yuri Shkuro <github@ysh.us>

data/indexpage.yaml

@@ -1,11 +1,11 @@
 features:
-- text: monitor and troubleshoot distributed workflows
-  icon: tv
-- text: identify performance bottlenecks
-  icon: build
+- text: monitor distributed workflows
+  icon: insights
+- text: find & fix performance bottlenecks
+  icon: speed
 - text: track down root causes
   icon: call_split
 - text: analyze service dependencies
-  icon: format_align_center
+  icon: account_tree
 why: |
   Distributed tracing observability platforms, such as Jaeger, are essential for modern software applications that are architected as microservices. Jaeger maps the flow of requests and data as they traverse a distributed system. These requests may make calls to multiple services, which may introduce their own delays or errors. Jaeger connects the dots between these disparate components, helping to identify performance bottlenecks, troubleshoot errors, and improve overall application reliability. Jaeger is 100% open source, cloud native, and infinitely scalable.

themes/jaeger-docs/layouts/partials/footer.html

@@ -28,7 +28,7 @@ <h4 class="is-size-4 is-size-3-mobile">
         {{ end }}
         <a href="https://cloud-native.slack.com/archives/CGG7NFUJ3">
           <span class="icon" style="color: #e03464;">
-            <i class="fab fa-slack"></i>
+            <i class="fab fa-lg fa-slack"></i>
           </span>
         </a>
         {{ with $mediumHandle }}