Clarify language on pull request review

[aduth]

This makes a few revisions to the Pull Request Process article to improve the clarity of the language:

• Be very clear about "one approving review" (vs. ambiguous "approval")
• Add note about potential exceptions for others merging a pull request (see recent example)
• Use full names instead of acronyms ("pull request" instead of "PR", "continuous integration" instead of "CI")
• Capitalize GitHub consistently

This was motivated partly by a recent discussion about our peer review procedure, and whether we have documented expectations around how approval works.

Live preview: https://federalist-7d0b2b76-42fc-4df1-9825-8c3cd77e0a15.sites.pages.cloud.gov/preview/gsa-tts/identity-handbook/aduth-pr-review-clarity/articles/pull-request-review.html