[flang] include snippet example to build FIR documentation

[lsandov1]

Summary

The new snippet is similar to the one provided by the doxygen-based
documentation, however no cmake cmd provided (no special flag is
required on the cmake step).

[lsandov1]

arc diffing with the correct commit ancestor

[kiranchandramohan]

Thanks for this patch. Please see comment inline.

flang/README.md

334

The LINK_WITH_FIR cmake variable was removed (https://reviews.llvm.org/D96411) and is not used or required now. You can remove the requirement of LINK_WITH_FIR and its usage in the snippet.

[lsandov1]

remove the camke '-DLINK_WITH_FIR=ON' reference on text and code snippet

[kiranchandramohan]

The documentation is also available in https://flang.llvm.org/docs/FIRLangRef.html.

flang/README.md

339

[lsandov1]

@kiranchandramohan I still have no power to merge, so please go ahead.

flang/README.md

 To run all of the flang regression tests use the `check-flang` target:
 ```bash
 ninja check-flang
 ```
 
 # How to Generate Documentation
 
 ## Generate FIR Documentation
 If flang was built with `-DLINK_WITH_FIR=ON` (`ON` by default), it is possible to
 generate FIR language documentation by running `ninja flang-doc`. This will
 create `<build-dir>/tools/flang/docs/Dialect/FIRLangRef.md` in flang build directory.
 
+```bash
+cd ~/llvm-project/build
+cmake -DLINK_WITH_FIR=ON ../llvm
+ninja flang-doc
+```
+
 ## Generate Doxygen-based Documentation
 To generate doxygen-style documentation from source code
 - Pass `-DLLVM_ENABLE_DOXYGEN=ON -DFLANG_INCLUDE_DOCS=ON` to the cmake command.
 
 ```bash
 cd ~/llvm-project/build
 cmake -DLLVM_ENABLE_DOXYGEN=ON -DFLANG_INCLUDE_DOCS=ON ../llvm
 ninja doxygen-flang