doc: remove personal pronoun guidance from style guide

In order to facilitate a more formal voice, the style guide recommends avoiding personal pronouns. In practice, this results in lots of nits for our docs without actually improving them. There are lots of ways our docs can be improved. Making them more formal seems low value, especially for all the PR comments this guidance generates.

So, remove that guidance from the style guide.

Checklist

• documentation is changed or added
• commit message follows commit guidelines