chore(space): add HF Space thumbnail + update README metadata

The Space's README.md frontmatter is what HF Spaces uses to render the social card on the OUTER URL (huggingface.co/spaces/AdithyaSK/rl-environments-guide). Without a `thumbnail:` field, HF auto-generates a default card via cdn-thumbnails.huggingface.co, which is what's currently being served. The article's inner `og:image` meta tag (set via seoThumbImage) doesn't apply here because social scrapers fetch the outer URL, not the iframe.

- title: refresh to match the article's current title.
- short_description: add for the Space card byline + og:description fallback.
- thumbnail: point at the GitHub raw URL on RL_Envs_101, no binary lives in the Space repo.
- tags: add rl-environments, llm-training, reinforcement-learning so the Space surfaces in HF filters.

README.md

@@ -1,5 +1,6 @@
 ---
-title: "The ultimate guide to LLM environments: How to scale to a 1000s of environments"
+title: "The ultimate guide to RL environments: building and scaling them in the LLM era"
+short_description: "Building and scaling RL environments for LLM training"
 emoji: 📝
 colorFrom: blue
 colorTo: indigo
@@ -9,6 +10,11 @@ header: mini
 pinned: false
 tags:
   - research-article-template
+  - rl-environments
+  - llm-training
+  - reinforcement-learning
+thumbnail: >-
+  https://raw.githubusercontent.com/adithya-s-k/RL_Envs_101/refs/heads/main/assets/blog_thumbnail.png
 ---
 
 # RL_environments