Add logo and banner (#277)

Following on from
#144, adds SVG
files for logo we decided on in meeting today to a new `images`
subdirectory plus a corresponding social banner to address task in #246.
The text colour on the banner should be visible on both dark and light
backgrounds.

I've also configured the Jekyll site to show the logo in the header bar
- by default this was very small so I've added some custom (S)CSS to
adjust the header div size and center the logo.

Currently only the logo is shown in the header not any text - not sure
if it is possible to override this?

Author: matt-graham

docs/_config.yml

@@ -6,3 +6,5 @@ url: https://UCL-ARC.github.io/python-tooling
 
 aux_links:
   Source repository: https://github.com/UCL-ARC/python-tooling
+
+logo: "https://raw.githubusercontent.com/UCL-ARC/python-tooling/main/images/logo.svg"

docs/_sass/custom/custom.scss

@@ -0,0 +1,8 @@
+div.site-header {
+  max-height: 15em;
+  height: 15em;
+}
+
+div.site-logo {
+  background-position: center center;
+}