Why:
* For BlockScout users to learn about our GraphQL API.
* Issue link: n/a
This change addresses the need by:
* Editing `_topnav.html.eex` template to include link to `/graphiql`.
* Fixing `/api_docs/index.html.eex` template to use "BlockScout" instead
of "Explorer".
* Updates translation/gettext files.
<p class="mt-4"><%= gettext "This API is provided for developers transitioning their applications from Etherscan to Explorer. It supports GET and POST requests." %></p>
<p class="mt-4"><%= gettext "This API is provided for developers transitioning their applications from Etherscan to BlockScout. It supports GET and POST requests." %></p>