Users reporting bad certificate on docs.gitlab.com

We have several Apple users reporting bad certificate (*.githost.io) and 404 when accessing https://docs.gitlab.com/.

This issue is to track and triage it.

Slack thread: https://gitlab.slack.com/archives/C101F3796/p1507831877000318

To reproduce the issue with openssl, one should disable SNI:

$> echo | openssl s_client -showcerts -servername docs.gitlab.com -connect docs.gitlab.com:443 2>/dev/null | openssl x509 -inform pem -noout -text | grep CN
        Issuer: C = GB, ST = Greater Manchester, L = Salford, O = COMODO CA Limited, CN = COMODO RSA Domain Validation Secure Server CA
        Subject: OU = Domain Control Validated, OU = PositiveSSL, CN = docs.gitlab.com

$> echo | openssl s_client -showcerts -connect docs.gitlab.com:443 2>/dev/null | openssl x509 -inform pem -noout -text | grep CN
        Issuer: C = BE, O = GlobalSign nv-sa, CN = AlphaSSL CA - SHA256 - G2
        Subject: OU = Domain Control Validated, CN = *.gitlab.io

/cc @gl-infra @connorshea