Feature Request: Rewrite markdown links

[jamesmunns]

In mnemos, we use relative links for many markdown files. It would be nice to:

• Rewrite these to oranda links, if they are to another README that is rendered in the workspace
• If not, rewrite to repository links, taking to account the folder structure

For example if I am in source/kernel/README.md, and I link to ../sermux-proto/README.md, that should go to the "sister project page", or if I write ../sermux-proto/src/lib.rs, that should go to the github page for that file.

At the moment, these links largely go to broken destinations.