Document custom RRF (#1985)

* make dedicated fusion strategy sections

* docs: Java, Go, C# snippets

Signed-off-by: Anush008 <anushshetty90@gmail.com>

* add formula for rrf

* add rust and python snippets

* Use consistent formatting

---------

Signed-off-by: Anush008 <anushshetty90@gmail.com>
Co-authored-by: Anush008 <anushshetty90@gmail.com>
Co-authored-by: Tim Visée <tim@visee.me>

qdrant-landing/content/documentation/headless/snippets/query-points/hybrid-rrf-k/rust.md

@@ -0,0 +1,13 @@
+```rust
+use qdrant_client::Qdrant;
+use qdrant_client::qdrant::{Rrf, PrefetchQueryBuilder, Query, QueryPointsBuilder};
+
+let client = Qdrant::from_url("http://localhost:6334").build()?;
+
+client.query(
+    QueryPointsBuilder::new("{collection_name}")
+        // .add_prefetch(...)  <┐
+        // .add_prefetch(...)  <┴─ 2+ prefetches here
+        .query(Query::new_rrf(RrfBuilder::with_k(60))
+).await?;
+```