move how-to up in documentation

src/fast_forward/docs/main.md

@@ -4,11 +4,20 @@ This is the implementation of [Fast-Forward indexes](https://dl.acm.org/doi/abs/
 
 # Features
 
-- Efficient look-up-based computation of semantic ranking scores
-- Interpolation of lexical (retrieval) and semantic (re-ranking) scores
+- **Efficient look-up-based computation** of semantic ranking scores
+- **Interpolation** of lexical (retrieval) and semantic (re-ranking) scores
 - Passage- and document-level ranking, including MaxP, FirstP, and AverageP
-- Early stopping for limiting index look-ups
-- Index compression via quantization and sequential coalescing
+- **Early stopping** for limiting index look-ups
+- Index compression via **quantization** and **sequential coalescing**
+
+# How to...
+
+- [create and use Fast-Forward indexes?](fast_forward/index.html)
+- [index a collection?](fast_forward/util.html#indexer)
+- [use quantization to reduce index size?](fast_forward/quantizer.html)
+- [create custom encoders?](fast_forward/encoder.html#custom-encoders)
+- [read, manipulate, and save rankings?](#rankings)
+- [use Fast-Forward indexes with PyTerrier?](fast_forward/util/pyterrier.html)
 
 # Installation
 
@@ -69,12 +78,3 @@ out = ff_index(first_stage_ranking)
 # interpolate scores and create a new TREC runfile
 first_stage_ranking.interpolate(out, 0.1).save(Path("/path/to/output/run.tsv"))
 ```
-
-## How to...
-
-- [create and use Fast-Forward indexes?](fast_forward/index.html)
-- [index a collection?](fast_forward/util.html#indexer)
-- [use quantization to reduce index size?](fast_forward/quantizer.html)
-- [create custom encoders?](fast_forward/encoder.html#custom-encoders)
-- [read, manipulate, and save rankings?](#rankings)
-- [use Fast-Forward indexes with PyTerrier?](fast_forward/util/pyterrier.html)