docs: drop the 'Add a Compute Backend' how-to (belongs in upstream SKaiNET docs)

Compute backends are an engine (sk.ainet.core) concern, not a transformers one;
the how-to (and its nav link) referenced a core skainet-backend-metal artifact and
duplicated upstream material. Removed the page and its nav entry.

Author: michalharakal

docs/modules/ROOT/nav.adoc

@@ -10,7 +10,6 @@
 
 .How-to Guides
 * xref:how-to/add-model.adoc[Add a New Model Architecture]
-* xref:how-to/add-compute-backend.adoc[Add a Compute Backend]
 * xref:how-to/add-tool.adoc[Add a Custom Tool]
 * xref:how-to/run-unified-cli.adoc[Use the Unified CLI]
 * xref:how-to/benchmarking.adoc[Run Benchmarks]