Restore missing environment variable content, plus minor fixes/updates

[guolau]

Affected Components

• Content & Marketing
• Pricing
• Test
• Docs
• Learn
• Other

Summary of changes

• Added some new built-in env vars that we hadn't listed previously
• Misc fixes for formatting, broken links, incorrect content etc.
• Restored missing content

Content from these old docs ...	... were added back to the new docs here
/docs/api-checks/variables/ Sections: Variables and secrets Managing variables	/platform/variables
/docs/api-checks/variables/ Sections: Accessing variables in API checks Using Handlebars helpers & built in variables	/detect/synthetic-monitoring/api-checks/requests
/docs/api-checks/setup-teardown-scripts/ Section: Generic runtime variables	/detect/synthetic-monitoring/api-checks/set-up-and-tear-down Nearly identical to old docs, except that one section.
/docs/browser-checks/variables Sections: Variables and secrets Managing variables Variable hierarchy	/platform/variables
/docs/browser-checks/variables Sections: Accessing variables Built-in runtime variables	/detect/synthetic-monitoring/browser-checks/mac-structure*
/docs/multistep-checks/ Sections: Built-in runtime variables	/detect/synthetic-monitoring/multistep-checks/overview*
/docs/groups/variables/	/platform/variables

*Not ideal. I'd like to restructure these sections and hopefully find a more suitable spot.

Related: https://linear.app/checklyhq/issue/DOC-19/missing-built-in-environment-variables-in-new-docs

[ragog]

@guolau just a small nit then we're good to go. That's important info you are bringing back.

[guolau]

Add link to CLI env vars page https://www.checklyhq.com/docs/cli/environment-variables/

Edit: done

[guolau]

Two small changes:

• Move "Accessing Environment Variables" in API docs (as requested by @ragog)
• Added link to CLI env vars page
  • 

[ragog]

@guolau LGTM

[ragog]

@guolau I'll take the liberty to merge since you are out.