config: add site description

Author: willcl-ark

_config.yml

@@ -21,9 +21,7 @@
 title: Bitcoin Core Onboarding
 email: [email protected]
 description: >- # this means to ignore newlines until "baseurl:"
-  Write an awesome description for your new site here. You can edit this
-  line in _config.yml. It will appear in your document head meta (for
-  Google search results) and in your feed.xml site description.
+  Bitcoin Core developer obboarding documentation
 baseurl: "" # the subpath of your site, e.g. /blog
 url: "https://bitcoincore.academy" # the base hostname & protocol for your site, e.g. http://example.com
 twitter_username: chaincodelabs