Add information on case sensitivity with Git Sync

[Rodrigo Castro]

It seems like Git Sync won't work as expected if you use uppercase letters in your folder structure.

@jpreynat has written a great explanation about it here: https://github.com/GitbookIO/support-bucket/issues/1204#issuecomment-1317304152

This is an intentional limitation, and we should include that in our docs to avoid frustration.