README: Remove "solutions" section. Add "explanations" section.

Author: amotl

README.md

@@ -9,14 +9,14 @@
 [Install](https://cratedb.com/docs/guide/install/) •
 [How-to guides](https://cratedb.com/docs/guide/howto/) •
 [Tutorials](https://cratedb.com/docs/guide/tutorial/) •
+[Explanations](https://cratedb.com/docs/guide/explain/) •
 [Administration](https://cratedb.com/docs/guide/admin/) •
 [Performance](https://cratedb.com/docs/guide/performance/)
 
 🔗 Reference material:
 [All features](https://cratedb.com/docs/guide/feature/) •
 [Ingestion](https://cratedb.com/docs/guide/ingest/) •
 [Topics](https://cratedb.com/docs/guide/topic/) •
-[Solutions](https://cratedb.com/docs/guide/solution/) •
 [Integrations](https://cratedb.com/docs/guide/integrate/)
 
 📖 More information:
@@ -70,14 +70,14 @@ navigation element (left-hand menu).
 - `connect` The canonical "How to connect to CrateDB" section.
 - `howto` How-to guides about CrateDB.
 - `tutorial` Tutorials about CrateDB.
+- `explain` Explanatory guides about CrateDB.
 - `admin` A potpourri of operational guidelines.
 - `performance` A potpourri of performance tuning guidelines.
 
 **Reference material**
 - `feature` A backbone section about all features at a glance, using a flat layout.
 - `ingest` A category section bundling all ingest methods.
 - `topic` A category section bundling different topics [sic!] and application domains.
-- `solution` A category section bundling different successful customer scenarios.
 - `integrate` A backbone section about all the integration items, using a flat layout.
 
 ## 💁 Contributing