Add docs for polling block handlers (#485)

* Add docs for polling block handlers

* Add spec version requirement for block handler filters

Author: incrypto32

website/pages/en/developing/creating-a-subgraph.mdx

@@ -770,6 +770,8 @@ In addition to subscribing to contract events or function calls, a subgraph may
 
 ### Supported Filters
 
+#### Call Filter
+
 ```yaml
 filter:
   kind: call
@@ -806,6 +808,45 @@ dataSources:
             kind: call
 ```
 
+#### Polling Filter
+
+> **Requires `specVersion` >= 0.8.0**
+
+> **Note:** Polling filters are only available on dataSources of `kind: ethereum`.
+
+```yaml
+blockHandlers:
+  - handler: handleBlock
+    filter:
+      kind: polling
+      every: 10
+```
+
+The defined handler will be called once for every `n` blocks, where `n` is the value provided in the `every` field. This configuration allows the subgraph to perform specific operations at regular block intervals.
+
+#### Once Filter
+
+> **Requires `specVersion` >= 0.8.0**
+
+> **Note:** Once filters are only available on dataSources of `kind: ethereum`.
+
+```yaml
+blockHandlers:
+  - handler: handleOnce
+    filter:
+      kind: once
+```
+
+The defined handler with the once filter will be called only once before all other handlers run. This configuration allows the subgraph to use the handler as an initialization handler, performing specific tasks at the start of indexing.
+
+```ts
+export function handleOnce(block: ethereum.Block): void {
+  let data = new InitialData(Bytes.fromUTF8('initial'))
+  data.data = 'Setup data here'
+  data.save()
+}
+```
+
 ### Mapping Function
 
 The mapping function will receive an `ethereum.Block` as its only argument. Like mapping functions for events, this function can access existing subgraph entities in the store, call smart contracts and create or update entities.