DNS active health checks for NGINX Plus

[y82]

Patch 1: Added procedure for DNS active health check.

• detailed configuration of active health checks for DNS
• intro and "see also" added
• a href and span html removed

Patch 2: Minor fixes in TCP/UDP load balancing

• broken link to nginx.com fixed
• span html tag converted to id tag
• API version updated to 9
• "See also" added

Tested locally with Hugo preview
Closes #401

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[ADubhlaoich]

LGTM!