Remove docs/Gemfile.lock and update github-pages version (facebook#11173)

Summary:
One system reports that a dependency in docs/Gemfile.lock is out-of-date and has a risk. I don't see a point of having Gemfile.lock checked in and dealing with dependencies all the time at all. It should be able to regenerated using `bundle install`. Update Gemfile file to a later version too.

Pull Request resolved: facebook#11173

Test Plan:
Run
bundle install
bundle exec jekyll serve --host=0.0.0.0
and see website working locally.

Reviewed By: ajkr

Differential Revision: D42897698

fbshipit-source-id: aeaf065c28b8f6582f1af1b5ffbbd5fa194afe24

Author: siying

docs/Gemfile

@@ -1,4 +1,4 @@
 source 'https://rubygems.org'
-gem 'github-pages', '~> 225'
+gem 'github-pages', '~> 227'
 
 gem "webrick", "~> 1.7"