Use the term "verification logic" consistently instead "verifiers"

Use cases, pain points, and background
When users are first introduced to concepts, core components, and terminology it's important to use consistent language. Right now, when you look at

• About NVIDIA NeMo Gym it says verifiers (logic to score performance)
• In the core abstractions it says Verification Logic: Scoring systems that evaluate agent responses for training/evaluation
• In the key terminology, verifier is used

Description:
Align on one term to use for consistency unless a different term must be used. We discussed that "verification logic" is a better term than "verifiers."

Design:

• About NVIDIA NeMo Gym
• core abstractions
• key terminology
• any other Github documentation that may reference this term. I didn't find any, but if information about Resource Servers are added via READMEs then consider this issue. There are some related documentation issues including #331 and #383

Out of scope:

• Changing anything in the source code to not use the word "verifiers"

Acceptance Criteria:

• Update About NVIDIA NeMo Gym to use "verification logic"
• Update key terminology to use "verification logic"

Start with the linked About NVIDIA NeMo Gym, core abstractions, and key terminology pages, then search the GitHub documentation for references to “verifiers.” Update the specified user-facing terminology to “verification logic,” leave source code unchanged, and verify both acceptance-criteria pages use the agreed term consistently.