Slightly improve favicon.ico.

Trim the whitespace so that the tube looks bigger while maintaining the aspect ratio.
Use File.exist? instead of deprecated File.exists?
Merge pull request #2213 from XhmikosR/docs-favicon
Update history to reflect merge of #2213 [ci skip]
Merge pull request #2214 from stomar/file-exists
Update history to reflect merge of #2214 [ci skip]
Adds reference to jekyll-page-collections plugin
Merge pull request #2215 from jeffkole/page-collection-plugin
Update history to reflect merge of #2215 [ci skip]