Improve "rustc_codegen_nvvm" chapter structure.

Currently section 6 is empty and only has one subsection. So this commit
moves the text in 6.1 into 6 and all of 6.1.n into 6.n.

Also, there are two sections (`ptxgen.md` and `deuugging.md`) that are
present but not shown because they aren't mentioned in `SUMMARY.md`.
Include them.

Author: nnethercote

guide/src/SUMMARY.md

@@ -13,7 +13,8 @@
   - [GPU Computing](cuda/gpu_computing.md)
   - [The CUDA Pipeline](cuda/pipeline.md)
 - [rustc_codegen_nvvm](nvvm/README.md)
-  - [Technical](nvvm/technical/README.md)
-    - [Custom Rustc Backends](nvvm/technical/backends.md)
-    - [rustc_codegen_nvvm](nvvm/technical/nvvm.md)
-    - [Types](nvvm/technical/types.md)
+  - [Custom Rustc Backends](nvvm/backends.md)
+  - [rustc_codegen_nvvm](nvvm/nvvm.md)
+  - [Types](nvvm/types.md)
+  - [PTX Generation](nvvm/ptxgen.md)
+  - [Debugging](nvvm/debugging.md)

guide/src/nvvm/README.md

@@ -0,0 +1,10 @@
+# rustc_codegen_nvvm
+
+This section will cover the more technical details of how rustc_codegen_nvvm works 
+as well as the issues that came with it.
+
+It will also explain some technical details about CUDA/PTX/etc, it is not necessarily 
+limited to rustc_codegen_nvvm.
+
+Basic knowledge of how rustc and LLVM work and what they do is assumed. You can find
+info about rustc in the [rustc dev guide](https://rustc-dev-guide.rust-lang.org/).