Fix Yellowstone API overview links copying URL instead of navigating (#1416)

* Fix Yellowstone overview links copying URL instead of navigating

The "Subscription types" entries were written as heading-links
(### [text](url)). Fern attaches a copy-section-anchor handler to
headings, which intercepted the click and copied the page anchor
instead of following the link to the subpage. Convert them to a
CardGroup so navigation works and the index reads cleaner.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Remove icons from Yellowstone overview cards

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: masterbd

content/api-reference/yellowstone/api-reference-overview.mdx

@@ -10,17 +10,24 @@ Yellowstone gRPC provides real-time streaming of Solana blockchain data through
 
 ## Subscription types
 
-### [Subscribe Request](/docs/reference/yellowstone-grpc-subscribe-request)
-The base structure for all subscriptions. Defines filters, commitment levels, and combines multiple subscription types in a single request.
-
-### [Subscribe to Slots](/docs/reference/yellowstone-grpc-subscribe-slots)
-Track slot progression and chain state. Essential for understanding confirmations and synchronizing with blockchain time.
-
-### [Subscribe to Transactions](/docs/reference/yellowstone-grpc-subscribe-transactions)
-Stream transactions with filters for accounts, vote/non-vote, and success/failure status.
-
-### [Subscribe to Accounts](/docs/reference/yellowstone-grpc-subscribe-accounts)
-Monitor account changes with filters for addresses, owners, data patterns, and balances.
-
-### [Subscribe to Blocks](/docs/reference/yellowstone-grpc-subscribe-blocks)
-Stream complete blocks with all transactions and metadata.
+<CardGroup>
+  <Card title="Subscribe Request" href="/docs/reference/yellowstone-grpc-subscribe-request">
+    The base structure for all subscriptions. Defines filters, commitment levels, and combines multiple subscription types in a single request.
+  </Card>
+
+  <Card title="Subscribe to Slots" href="/docs/reference/yellowstone-grpc-subscribe-slots">
+    Track slot progression and chain state. Essential for understanding confirmations and synchronizing with blockchain time.
+  </Card>
+
+  <Card title="Subscribe to Transactions" href="/docs/reference/yellowstone-grpc-subscribe-transactions">
+    Stream transactions with filters for accounts, vote/non-vote, and success/failure status.
+  </Card>
+
+  <Card title="Subscribe to Accounts" href="/docs/reference/yellowstone-grpc-subscribe-accounts">
+    Monitor account changes with filters for addresses, owners, data patterns, and balances.
+  </Card>
+
+  <Card title="Subscribe to Blocks" href="/docs/reference/yellowstone-grpc-subscribe-blocks">
+    Stream complete blocks with all transactions and metadata.
+  </Card>
+</CardGroup>