Fix namespace, identifier-type and system per issue #773

[Rene2mt]

This PR makes website documentation updates to resolve issue GSA/fedramp-automation/issues/773

Specifically, it makes the following changes throughout the site:

• For FedRAMP extension properties, changes namespace from "https://fedramp.gov/ns/oscal" to "http://fedramp.gov/ns/oscal"
• For the SSP identifier-type, changes the value from "https://fedramp.gov/ns/oscal" to "http://fedramp.gov/ns/oscal"
• For POA&M facets, changes the system flag from "https://fedramp.gov" to "http://fedramp.gov/ns/oscal"