Fix docs building guide (#5825)

Update instructions with webrick dependency
Restore Gemfile that was accidentally removed in #5821

---------

Co-authored-by: Logan Adams <loadams@microsoft.com>

docs/Gemfile

@@ -0,0 +1,24 @@
+source "https://rubygems.org"
+
+gem 'github-pages', group: :jekyll_plugins
+
+# If you have any plugins, put them here!
+group :jekyll_plugins do
+  gem "jekyll-feed"
+  gem "jekyll-paginate"
+  gem "jekyll-remote-theme"
+  gem "jekyll-include-cache"
+  gem "minimal-mistakes-jekyll"
+end
+
+# Windows and JRuby does not include zoneinfo files, so bundle the tzinfo-data gem
+# and associated library.
+install_if -> { RUBY_PLATFORM =~ %r!mingw|mswin|java! } do
+  gem "tzinfo", "~> 1.2"
+  gem "tzinfo-data"
+end
+
+# Performance-booster for watching directories on Windows
+gem "wdm", "~> 0.1.1", :install_if => Gem.win_platform?
+
+gem "webrick", "~> 1.8"

docs/README.md

@@ -42,6 +42,16 @@ We now need to install the required Ruby packages for the website.
 bundle install
 ```
 
+Depending on your environment, you may need to add `webrick` to avoid the following [error](https://talk.jekyllrb.com/t/load-error-cannot-load-such-file-webrick/5417/6):
+
+> gems/gems/jekyll-3.9.5/lib/jekyll/commands/serve/servlet.rb:3:in `require': cannot load such file -- webrick (LoadError)
+
+
+```
+bundle add webrick
+```
+
+
 You can now start a local webserver via:
 ```
 bundle exec jekyll serve