Generalize W3C Digital Credentials API references

[timcappalli]

Based on the discussion on yesterday's call, this changes the following:

• Adds a new term in Terminology for "The Digital Credentials API" which talks about both the Web Platform API (W3C) and the native platform equivalents (generically)
• Changes most occurrences of "W3C Digital Credentials API" to either "Digital Credentials API (DC API)" or "DC API"
• Changes the response_mode to dc_api
• Includes "app origin" along with web origin
• Fixes xref for W3C DC API