Accept use of common contractions

What does this MR do?

In an effort to create a more informal tone for GitLab docs, this MR modifies the current prohibition on contractions in the documentation style guide

I'm proposing a change to allow (and even encourage) "common" contractions where it enhances a conversational tone. (At Mike Lewis' suggestion, this MR would keep the prohibition for "reference" docs.) The model I'm using is the Microsoft Manual of Style, specifically the section entitled use contractions.

I note that they also have comments on contractions in their UI. Contrast with our guidance in the UX Pajamas Guide that supports the use of contractions when it makes "writing simpler and easier to read". I personally agree with that guidance, cc @tauriedavis , and have therefore linked to it in this MR.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

n/a