[Flang][Docs] Add Sphinx man page support for Flang

This patch enables building Flang man pages by:

- Adding a `man_pages` entry in flang/docs/conf.py for Sphinx man builder.
- Adding a minimal `index.rst` as the master document.
- Adding placeholder `.rst` files for FIRLangRef and FlangCommandLineReference to fix toctree references.

These changes unblock builds using `-DLLVM_BUILD_MANPAGES=ON` and allow `ninja docs-flang-man` to generate `flang.1`.

Fixes #141757

Author: snarang181

flang/docs/FIRLangRef.rst

@@ -0,0 +1,4 @@
+FIR Language Reference
+======================
+
+(TODO: Add FIR language reference documentation)

flang/docs/FlangCommandLineReference.rst

@@ -0,0 +1,4 @@
+Flang Command Line Reference
+============================
+
+(TODO: Add Flang CLI documentation)

flang/docs/conf.py

@@ -227,7 +227,9 @@
 
 # One entry per manual page. List of tuples
 # (source start file, name, description, authors, manual section).
-man_pages = []
+man_pages = [
+    ('index', 'flang', 'Flang Documentation', ['Flang Contributors'], 1)
+]
 
 # If true, show URL addresses after external links.
 # man_show_urls = False

flang/docs/index.rst

@@ -0,0 +1,10 @@
+Flang Documentation
+====================
+
+Welcome to the Flang documentation.
+
+.. toctree::
+   :maxdepth: 1
+
+   FIRLangRef
+   FlangCommandLineReference