Update guides generation to use Nokogiri's HTML5 parser

[flavorjones]

Motivation / Background

Use Nokogiri::HTML5 to generate the guides. This doesn't meaningfully change the output, but will help ensure we're emitting valid HTML5 markup. A followup commit could reasonably rip out the W3C validator, I think.

Note that the most frequent changes to the output are:

• attribute values, most often data-clipboard-text. TheHTML5 spec prescribes entity-escaping fewer characters in attribute values than libxml2 did, and wraps attribute values in double-quotes. In particular > and < are not escaped per the HTML5 spec which may be surprising to look at if you're used to libxml2's output.
• linebreaks are different for some HTML elements, particularly lists and tables have fewer linebreaks between elements.

Detail

This Pull Request replaces calls to Nokogiri::HTML.fragment with a call to a new private method, html_fragment which uses either Nokogiri::HTML5.fragment if HTML5 available, else Nokogiri::HTML4.fragment.

Additional information

Part of an ongoing effort to update Rails to use HTML5.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.