docs: re-add documentation for SSL_CERT_FILE and SSL_CERT_DIR

[tobb10001]

The section in environment_variables.md is taken from 3f7657, which was the last commit before it was mistakenly removed.

Summary

Re-add the documentation for SSL_CERT_FILE and SSL_CERT_DIR, as discussed in #3570.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change. doesn't apply
• I've updated the documentation accordingly.

[tobb10001]

I would like to bring to your attention, that while working on this, I noticed that mkdocs is emitting some warnings for unresolved links on build, which appear to be unrelated to the changes here:

$ ./scripts/docs
+ mkdocs serve
INFO    -  Building documentation...
INFO    -  Cleaning site directory
INFO    -  Doc file 'async.md' contains an unrecognized relative link '../advanced/transports#asgitransport', it was left as is. Did you mean 'advanced/transports.md#asgitransport'?
INFO    -  Doc file 'compatibility.md' contains a link 'advanced/clients.md#client-instances', but the doc 'advanced/clients.md' does not contain an anchor '#client-instances'.
INFO    -  Doc file 'environment_variables.md' contains a link 'advanced/proxies.md#http-proxying', but the doc 'advanced/proxies.md' does not contain an anchor '#http-proxying'.
INFO    -  Doc file 'third_party_packages.md' contains a link 'advanced/authentication.md#customizing-authentication', but the doc 'advanced/authentication.md' does not contain an anchor
           '#customizing-authentication'.
INFO    -  Doc file 'advanced/clients.md' contains a link '#client-instances', but there is no such anchor on this page.
INFO    -  Doc file 'advanced/clients.md' contains a link '#merging-of-parameters', but there is no such anchor on this page.
INFO    -  Doc file 'advanced/proxies.md' contains a link '#routing', but there is no such anchor on this page.
INFO    -  Documentation built in 1.94 seconds

If you want I can try to resolve those in another PR.