Modify build script to use HTML routes

Author: Samarth Narang

flang/docs/FIR/CreateFIRLangRef.py

@@ -4,9 +4,9 @@
 import os
 
 # These paths are relative to flang/docs in the build directory, not source, as that's where this tool is executed.
-HEADER_PATH = os.path.join("Source", "FIR", "FIRLangRef_Header.md")
+HEADER_PATH = os.path.join("SourceHtml", "FIR", "FIRLangRef_Header.md")
 DOCS_PATH = os.path.join("Dialect", "FIRLangRef.md")
-OUTPUT_PATH = os.path.join("Source", "FIRLangRef.md")
+OUTPUT_PATH = os.path.join("SourceHtml", "FIRLangRef.md")
 
 # 1. Writes line 1 from docs to output, (comment line that the file is autogenerated)
 # 2. Adds a new line