6980: Implement to support intra-doc link r=matklad a=sasurau4 Helps with #6168 This PR is very limited implementation to support intra-doc. It only support links indicate same file function. I want someone to feedback me about this implementation. If the approach is good, I will continue this PR to support other symbols like enum and struct. Co-authored-by: Daiki Ihara <sasurau4@gmail.com>