fix(docs): pin gym doc links to v0.2.1 instead of blanket-ignoring

Replace the blanket linkcheck_ignore for all NeMo Gym docs with
pinned v0.2.1 URLs so linkcheck still validates them.

Signed-off-by: Terry Kong <terryk@nvidia.com>

Author: terrykong

docs/conf.py

@@ -117,7 +117,6 @@
 linkcheck_ignore = [
     ".*github\\.com.*",
     ".*githubusercontent\\.com.*",
-    "https://docs\\.nvidia\\.com/nemo/gym/.*",
 ]
 
 # PyTorch docs anchor IDs change between stable versions; verify the page

docs/design-docs/nemo-gym-integration.md

@@ -1,6 +1,6 @@
 # NeMo Gym Integration
 
-This document describes how NeMo RL integrates with [NeMo Gym](https://docs.nvidia.com/nemo/gym/latest/index.html) for multi-step and multi-turn reinforcement learning training.
+This document describes how NeMo RL integrates with [NeMo Gym](https://docs.nvidia.com/nemo/gym/v0.2.1/index.html) for multi-step and multi-turn reinforcement learning training.
 
 ## Overview
 
@@ -181,7 +181,7 @@ sequenceDiagram
     GRPO->>Policy: Compute loss and train
 ```
 
-> **NeMo Gym server types** (see [Core Components](https://docs.nvidia.com/nemo/gym/latest/about/concepts/core-components.html)):
+> **NeMo Gym server types** (see [Core Components](https://docs.nvidia.com/nemo/gym/v0.2.1/about/concepts/core-components/)):
 > - **Agent Server**: Orchestrates the rollout loop
 > - **Model Server**: HTTP proxy to vLLM; translates Responses API ↔ Chat Completions
 > - **Resource Server**: Provides tools and rewards
@@ -254,4 +254,4 @@ Token IDs are extracted at the NeMo RL vLLM layer via the `/tokenize` endpoint.
 - Tokenization matches the exact model and tokenizer used for generation
 - No re-tokenization drift between generation and training
 
-For details on on-policy token ID handling, see {doc}`../guides/environments` and the [NeMo Gym on-policy corrections documentation](https://docs.nvidia.com/nemo/gym/latest/contribute/rl-framework-integration/openai-compatible-http-server-on-policy-correction.html).
+For details on on-policy token ID handling, see {doc}`../guides/environments` and the [NeMo Gym on-policy corrections documentation](https://docs.nvidia.com/nemo/gym/v0.2.1/contribute/rl-framework-integration/openai-compatible-http-server-on-policy-correction.html).