[Docs] Allow building man pages without myst_parser (#82402)

The man pages do not depend on the doc present in markdown files, so
they can be built without myst_parser.
Doing so might allow llvm distributions to have less development
dependencies.

As we do not have the ennvironment to test these configuration, this
capability is provided on a best-effort basis.

This restores a capability accidentally lost in #65664.

clang/docs/conf.py

@@ -35,8 +35,17 @@
 
 import sphinx
 
-if sphinx.version_info >= (3, 0):
+# When building man pages, we do not use the markdown pages,
+# So, we can continue without the myst_parser dependencies.
+# Doing so reduces dependencies of some packaged llvm distributions.
+try:
+    import myst_parser
+
     extensions.append("myst_parser")
+except ImportError:
+    if not tags.has("builder-man"):
+        raise
+
 
 # The encoding of source files.
 # source_encoding = 'utf-8-sig'

flang/docs/conf.py

@@ -22,13 +22,24 @@
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 extensions = [
-    "myst_parser",
     "sphinx.ext.todo",
     "sphinx.ext.mathjax",
     "sphinx.ext.intersphinx",
     "sphinx.ext.autodoc",
 ]
 
+# When building man pages, we do not use the markdown pages,
+# So, we can continue without the myst_parser dependencies.
+# Doing so reduces dependencies of some packaged llvm distributions.
+try:
+    import myst_parser
+
+    extensions.append("myst_parser")
+except ImportError:
+    if not tags.has("builder-man"):
+        raise
+
+
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ["_templates"]
 myst_heading_anchors = 6

llvm/docs/conf.py

@@ -26,7 +26,18 @@
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ["myst_parser", "sphinx.ext.intersphinx", "sphinx.ext.todo"]
+extensions = ["sphinx.ext.intersphinx", "sphinx.ext.todo"]
+
+# When building man pages, we do not use the markdown pages,
+# So, we can continue without the myst_parser dependencies.
+# Doing so reduces dependencies of some packaged llvm distributions.
+try:
+    import myst_parser
+
+    extensions.append("myst_parser")
+except ImportError:
+    if not tags.has("builder-man"):
+        raise
 
 # Automatic anchors for markdown titles
 from llvm_slug import make_slug