Remove tutorial, style guide and references

The compiler user guide should not contain a tutorial.

The style guide is very outdated.

The references are already on the main clash-lang.org site.

Author: DigitalBrains1

compiler-userguide/src/SUMMARY.md

@@ -4,7 +4,6 @@
 
 - [Getting Started]()
     - [Installing](./getting-started/installing.md)
-    - [First Circuit](./getting-started/first-circuit.md)
 - [General](./general/index.md)
     - [Release Notes](./general/relnotes.md)
     - [FAQs](./general/faqs.md)
@@ -14,5 +13,3 @@
     - [Prelude](./developing-hardware/prelude.md)
     - [Flags](./developing-hardware/flags.md)
 - [Hacking on Clash](./hacking-on-clash/index.md)
-    - [Style](./hacking-on-clash/style.md)
-- [References](./refs.md)