Increase slangc option visibility in docs (shader-slang#5425)

* Increase slangc option visibility in docs

Link to slangc command-line reference from readme.
readme. Boost the visiblity of the link in the user-guide.

Consolidates command-line-slangc.md into the user-guide.

Fixes shader-slang#4124

* Update README.md

* Update 08-compiling.md

---------

Co-authored-by: Yong He <yonghe@outlook.com>

Author: cheneym2

README.md

@@ -53,6 +53,9 @@ Slang binaries are also included in the [Vulkan SDK](https://vulkan.lunarg.com/s
 There are packages built for 32- and 64-bit Windows, as well as 64-bit Ubuntu.
 Each binary release includes the command-line `slangc` compiler, a shared library for the compiler, and the `slang.h` header.
 
+See the user-guide for info on using the `slangc` command-line tool: [Slang Command Line Usage](
+https://shader-slang.com/slang/user-guide/compiling.html#command-line-compilation-with-slangc).
+
 If you want to try out the Slang language without installing anything, a fast and simple way is to use the [Slang Playground](https://shader-slang.com/slang-playground). The playground allows you to compile Slang code to a variety of targets, and even run some simple shaders directly within the browser. The playground loads Slang compiler to your browser and runs all compilation locally. No data will be sent to any servers.
 
 If you would like to build Slang from source, please consult the [build instructions](docs/building.md).

docs/README.md

@@ -9,7 +9,7 @@ Getting Started
 
 The Slang [User's Guide](https://shader-slang.github.io/slang/user-guide/) provides an introduction to the Slang language and its major features, as well as the compilation and reflection API.
 
-There is also documentation specific to using the [`slangc`](command-line-slangc.md) command-line tool.
+There is also documentation specific to using the [slangc](https://shader-slang.github.io/slang/user-guide/compiling.html#command-line-compilation-with-slangc) command-line tool.
 
 Advanced Users
 --------------

docs/command-line-slangc.md

@@ -63,7 +63,7 @@ The astute reader may have noticed that the default Slang HLSL prelude *does* co
 #endif
 ```
 
-This means that the *downstream* compiler (such as DXC and FXC) must be able to handle this include. Include paths can be specified for downstream compilers via the [-X mechanism](command-line-slangc.md#downstream-arguments). So for example...
+This means that the *downstream* compiler (such as DXC and FXC) must be able to handle this include. Include paths can be specified for downstream compilers via the [-X mechanism](user-guide/08-compiling.md#downstream-arguments). So for example...
 
 ```
 -Xfxc -IpathTo/nvapi -Xdxc -IpathTo/nvapi