doc: move `cmake -B build -LH` up in Unix build docs #33088

#32269 rebased.

I had trouble building bitcoin core the way I wanted since now more features require a flag while building. IMO it makes sense to make it a bit more prominent in the build docs how to get the needed flags.

Related issue: https://github.com/bitcoin/bitcoin/issues/32258

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33088.

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ACK 6757052fc439bedd1fa88ee1d23b4f17cf2c4f7a

For all other platforms, this configure help lives close to the cmake --build build command, which makes most sense to me. I did note that build-windows.md and build-windows-msvc.md documentation does not provide the cmake -B build -LH help at all. If relevant, could be added in this PR? No blocker.

Example diff (untested):

<details> <summary>git diff on 6757052fc4</summary>

diff --git a/doc/build-windows-msvc.md b/doc/build-windows-msvc.md
index 2d85d223e9..7c07a19850 100644
--- a/doc/build-windows-msvc.md
+++ b/doc/build-windows-msvc.md
@@ -50,6 +50,8 @@ CMake will put the resulting object files, libraries, and executables into a ded
 
 In the following instructions, the "Debug" configuration can be specified instead of the "Release" one.
 
+Run `cmake -B build -LH` to see the full list of available options.
+
 ### 4. Building with Static Linking with GUI
 
 ```
diff --git a/doc/build-windows.md b/doc/build-windows.md
index 84db7fa4b5..24c6d06c96 100644
--- a/doc/build-windows.md
+++ b/doc/build-windows.md
@@ -47,9 +47,16 @@ This means you cannot use a directory that is located directly on the host Windo
 
 Build using:
 
+```bash
     gmake -C depends HOST=x86_64-w64-mingw32  # Append "-j N" for N parallel jobs.
     cmake -B build --toolchain depends/x86_64-w64-mingw32/toolchain.cmake
+```
+
+Run `cmake -B build -LH` to see the full list of available options.
+
+```bash
     cmake --build build     # Append "-j N" for N parallel jobs.
+```
 
 ## Depends system
 

</details>

lgtm ACK 6757052fc439bedd1fa88ee1d23b4f17cf2c4f7a

ACK 6757052fc439bedd1fa88ee1d23b4f17cf2c4f7a

This PR moves information on getting all the available options of cmake -B build to a more prominent place, line 13. The PR seems to me a logical change and makes me wonder why this information was at the end of the documentation in the first place.

@stickies-v might follow up with any Windows doc changes.

(might make sense to mention ccmake too?)