[DOCS] No supporting documentation exists for use of http_proxy / https_proxy

Description
Users particularly in air-gapped environments may require use of proxy servers, and no documentation exists to provide guidance in the usage of http_proxy or https_proxy settings to facilitate this.

Google searches for things like 'saltstack http proxy' return almost exclusively results for proxy minion behavior and are unlikely to locate the minion configuration page that contains the relevant opts.

Suggested Fix
Though minion configuration docs list the associated opts from minion perspective, there is no treatment of equivalent env vars http_proxy and https_proxy and their effect on minion and master behavior.

Type of documentation
Salt documentation

Location or format of documentation
https://docs.saltproject.io/en/latest/ref/configuration/minion.html
https://docs.saltproject.io/en/latest/ref/configuration/master.html

Additional context
See
https://github.com/saltstack/salt/pull/29322
https://github.com/saltstack/salt/issues/8177
[It is preposterously difficult to find reasonable documentation on use of http_proxy in env, but] https://www.cyberciti.biz/faq/linux-unix-set-proxy-environment-variable/

Start with the minion and master configuration pages named in the issue, then review pull request 29322 and issue 8177 for existing behavior and terminology. Document how http_proxy and https_proxy affect minion and master behavior, with guidance discoverable from both configuration pages.