Document Pantheon-SKey in addition to Pantheon Debug

[stevector]

Through this PR I learned about Pantheon-SKey https://github.com/pantheon-systems/documentation/pull/8775

Expected behavior

• We have reference material that covers all headers and/HTTP nuances that change the behavior of the platform.

Actual behavior

• Pantheon-SKey is only mentioned in Decouple Kit docs

Suggested Resolution

• Document Pantheon-SKey in the same places we document Pantheon-Debug

Additional information

Should we have a Reference Page (https://diataxis.fr/) that covers all things HTTP @rachelwhitton ?