Added flag for handling index.md generation with 1 file in folder in DocFxTocGenerator

[mtirionMSFT]

Changes to the DocFxTocGenerator tool:

• Standard behavior is still: if no INDEX.md or README.md is found, an INDEX.md is added to a folder (-i flag)
• Adding the -n flag to the -i will prevent generating an INDEX.md if there is only 1 file in the folder. In that case, that file is used as the reference for a folder.
• Changed to way to get the hierarchy of folders and files to improve performance.