This patch refactors the Flang documentation CMake and Sphinx
configuration to address build issues.
**CMake changes**:
- Moves the `gen_rst_file_from_td()` call out of the HTML-only block so
that both `docs-flang-html` and `docs-flang-man` builds depend on the
generated `FlangCommandLineReference.rst` file.
**conf.py changes**:
- Introduces `myst_parser` dependency as a required Markdown parser for
both HTML and man builds.
- Introduces the correct source_suffix mapping for both .rst and .md
files.
- Populates the man_pages configuration so the main index page generates
a ` flang(1) `man page.
Fixes#141757
---------
Authored-by: Samarth Narang <samanara@qti.qualcomm.com>
47171ac3f9