[OpenMP][docs] Revise building manual

[Meinersbur]

Go into more detail on the two non-legacy build modes and consolidate build instructions from different files into a dedicated document. index.rst only keeps the "Getting Started" part. README.rst is vastly outdated with the still valid parts integrated into the new documentation. SupportAndFAQ.rst is unstructured and keep only the non-building parts. Make the doxygen-integrated instructions link to the web resource.

The the new building document is written in Meltdown, following the decision from LLVM.

[Meinersbur]

I think updating the documentation not as a single person's job, but the community's task. If you would like changes or addition, please always propose the actual text you would like to see.

[github-actions]

✅ With the latest revision this PR passed the Python code formatter.

[github-actions]

⚠️ Python code formatter, darker found issues in your code. ⚠️

You can test this locally with the following command:

darker --check --diff -r origin/main...HEAD openmp/docs/conf.py

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View the diff from darker here.

--- conf.py	2026-01-15 14:35:05.000000 +0000
+++ conf.py	2026-01-15 14:36:44.363545 +0000
@@ -24,11 +24,16 @@
 # If your documentation needs a minimal Sphinx version, state it here.
 # needs_sphinx = '1.0'
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ["sphinx.ext.todo", "sphinx.ext.mathjax", "sphinx.ext.intersphinx", "myst_parser"]
+extensions = [
+    "sphinx.ext.todo",
+    "sphinx.ext.mathjax",
+    "sphinx.ext.intersphinx",
+    "myst_parser",
+]
 
 # Add path for llvm_slug module.
 sys.path.insert(0, os.path.abspath(os.path.join("..", "..", "llvm", "docs")))
 
 

[jhuber6]

Very good cleanup and simplification.

I also didn't know you could mix markdown and restructured text like this

[Meinersbur]

I also didn't know you could mix markdown and restructured text like this

LLVM's docs do so. It would probably be infeasable to convert all ReST to Markdown at once if this was not the case: https://reviews.llvm.org/D44910

[shiltian]

Thanks for the detailed documents!

[pkl97]

@Meinersbur: May I draw your attention to issue #174701?

There I ran into an OpenMP build error when CLANG_DEFAULT_CXX_STDLIB=libc++ is used while building OpenMP as a runtime. Will there be a solution mentioned for this issue in your planned OpenMP build documentation?

The build process worked fine as long as OpenMP was built as a project (which is no longer an option after #152189 was merged).

[Meinersbur]

@pkl97 The documentation is not supposed to be a bug tracker. Please keep the discussion of an issue to that issue #.