API pages cleanup

[kbatuigas]

Description

• Ensure consistent usage of CIRCLE_TOKEN env var
• Ensure consistent usage of double quotes with env vars in curl commands as well as request format
• General content refresh

Reasons

Jira ticket

Content Checklist

Please follow our style when contributing to CircleCI docs. Our style guide is here: https://circleci.com/docs/style/style-guide-overview.

Please take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs) 😸:

• [x] Break up walls of text by adding paragraph breaks.
• [x] Consider if the content could benefit from more structure, such as lists or tables, to make it easier to consume.
• [x] Keep the title between 20 and 70 characters.
• [x] Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
• [x] Check all headings h1-h6 are in sentence case (only first letter is capitalized).
• [x] Is there a "Next steps" section at the end of the page giving the reader a clear path to what to read next?
• [x] Include relevant backlinks to other CircleCI docs/pages.