Support linking to other slides

[MartenBE]

If not absolute path, and ends on .md: replace the extension with .html. Perhaps something similar to the nav option in mkdocs would be great.

[jaywhj]

Yes, this is necessary. If the markdown document is associated with other markdown files, the associated md files will be copied to the built directory during the build. This is actually not necessary. You only need to change the suffix to html.