Replace deprecated Ruby methods

Replace `Hash#has_key?` and `File.exists?` with `Hash#key?`
and `File.exist?`
add warning flag to note
fix warning flag (move from wrong note)
add back info class
improving strftime call

set non padded config to avoid parse string type twice
Explain where Main files go and whereone can find an example site.
Merge pull request #2669 from budparr/master
Update history to reflect merge of #2669 [ci skip]
Update site history.
Add quick troubleshooting docs for 'Could not find a JavaScript runtime.'

Fixes #2327.
change links from Markdown to HTML
fix quote
Merge pull request #2679 from budparr/master
Update history to reflect merge of #2679 [ci skip]
Merge pull request #2673 from lucianosousa/patch-1
Update history to reflect merge of #2673 [ci skip]
Merge pull request #2664 from alfredxing/replace-deprecated-methods
Update history to reflect merge of #2664 [ci skip]
Add helpful comments to site template _config.yml
Add friendly error message for invalid dates

Catch the `ArgumentError` when parsing dates from the YAML front
matter