introduce HTTPX_CA_BUNDLE, fix docs

[Yanpas]

trafficstars

Summary

HTTPX_CA_BUNDLE env-variable was introduced (similar to REQUESTS_CA_BUNDLE). The motivation behind it: SSL_CERT_FILE affects other libraries/programs.

Alternatively I can introduce HTTPX_SYSTEM_CA.

Related to #302

Checklist

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