Improve feed.xml.

* add `generator` property
* add image property
* use `xml_escape` instead of `![CDATA]`
* use `date_to_rfc822` instead of the date format string (thanks @troyswanson!)
* fix self reference url
* switch to `isPermaLink` for the `guid`
Add meta generator property in the HTML files.
Merge pull request #2192 from XhmikosR/docs-feed
Update history to reflect merge of #2192 [ci skip]
Move vendor prefixed properties before the unprefixed ones.
Use the shorthand font property when possible.

Conflicts:
	site/_includes/css/style.css
Be explicit with the `background` property.
Remove units from zero.
Remove duplicate properties.
Add fallback colors.
Add missing vendor prefixed properties.
Simplify selectors.