Reorganize env vars documentation

[kbatuigas]

Description

Splits out the Environment variables page into a few standalone pages:

• Environment Variables (kept from original, mostly conceptual content only)
• Set an Environment Variable (steps for different methods of setting an env var in CircleCI)
• Inject Environment Variables with the CircleCI API
• Built-in Environment Variables (reference doc)

This PR also adds the new pages to the left side navigation, and updates references throughout other doc pages so that they point to the correct new page, where applicable.

Reasons

The original env vars document is very long and contains a mix of conceptual, how-to, and reference content. The goal of splitting the content up into different pages is to improve readability and help readers find the information they need more quickly.

Jira ticket (with visits metrics)

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.