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>