Upgrade sdoc to 2.1.0

Sdoc 2.1.0 works better on mobile and has improvements for SEO and
Lighthouse.

Changelog:
* rails#154 Make panel responsive for mobile.
* rails#153 Add viewport metatag to views for improved Lighthouse score.
* rails#150 Use semantic headers for better SEO.

Author: p8

Gemfile

@@ -36,7 +36,7 @@ group :rubocop do
 end
 
 group :doc do
-  gem "sdoc", ">= 2.0.4"
+  gem "sdoc", ">= 2.1.0"
   gem "redcarpet", "~> 3.2.3", platforms: :ruby
   gem "w3c_validators", "~> 1.3.6"
   gem "kindlerb", "~> 1.2.0"

Gemfile.lock

@@ -462,7 +462,7 @@ GEM
       sprockets (> 3.0)
       sprockets-rails
       tilt
-    sdoc (2.0.4)
+    sdoc (2.1.0)
       rdoc (>= 5.0)
     selenium-webdriver (4.0.0.alpha7)
       childprocess (>= 0.5, < 5.0)
@@ -615,7 +615,7 @@ DEPENDENCIES
   rubocop-performance
   rubocop-rails
   sass-rails
-  sdoc (>= 2.0.4)
+  sdoc (>= 2.1.0)
   selenium-webdriver (>= 4.0.0.alpha7)
   sequel
   sidekiq