New doc describing TLS update and how to check secure connections

[jazzsequence]

Priority: Medium

Issue Description

Pantheon is going to be pushing a TLS encryption update (updating to TLS encryption and blocking TLS connections < 1.2). There isn't a single, simple user-centric "fix" for this — users must check any sites where they are sourcing resources (e.g. third-party API endpoints or URLs) and confirm that they support the heightened encryption level. Note: This does not apply to scripts, CSS files, fonts, etc. sourced from external providers (and rendered by the browser on the client). This only applies to connections the Pantheon appserver is executing in code.

This will not affect most customers. But, because it has the potential to break sites, we would like to announce the change early (and often) to give people the opportunity to validate and test that their sites will not have any issues before the change is rolled out.

Therefore, a docs page should exist that discusses what TLS encryption is, how to test TLS encryption levels for URLs, and possibly how to even identify what to test.

This page can then be linked to from any other announcements, e.g. Status Page, in-dashboard banner, etc.

How will this impact users?

This will provide a resource that users can use and refer back to to ensure that their site code is compatible with TLS 1.2+ connections.